Keep them away from each. Don’t have him over to stay the night. Don’t let her stay the night at his home. Don’t let them be staying the night at their grandparents house at the same time. I say these things because I had a male cousin also one year older than me that groomed me from a young age (around the age she is now) and then just before I turned 12 began assaulting me and this went on for around 4-5 years, he talked about how he loved me and wanted to be with me along with all of this. I had also gone through csa prior to this and was very vulnerable and desperately wanted to feel loved. When I finally told my parents (mom and step dad) and they were angry at me. Grounded me, took all of my clothes that kept me warm (this was in the middle of winter, I only got to have them for the weekends that I had visitation with my dad), took my phone, and I wasn’t allowed to use any computer or tablet at all even for homework (which caused issues for my school work and grades leading to them getting even more upset at me). They told me over and over that it was my fault he did those things and that when you look for someone to date it’s fine to find someone that’s nice like him but you don’t date your actual cousin (due to his grooming me, etc he had convinced me that we were in a relationship). Sorry for such a long reply. I hope you are able to get through to your sister about this, my cousin got no consequences for any of what he did to me and I wasn’t allowed to let any other family members know what happened so going to family get togethers was really bad for my mental health. Also the way I got evidence to show my mom was texting him and asking why he liked me, his exact reply “you’re my fetish” I showed that text to my mom and yet everything still went the way it did.
Keep them apart; he needs therapy. They both do. Chances are, someone has already hurt HIM too
The best thing to do is act like no big deal… Just say not playing with each other’s parts
Id be worried about where he learned that…
Some one has done sometging to him or he has seen it… U and other momma deff need to sit down with both kids… Dont make it like they are in trouble tho… Poor babies
Keep him away… Talk to his mama… Help her find out who is doing it to him… Also confront him… Let him know u kno whats going on an u wont tolerate it… Dont be afraid to say something!!
We have safety zones “no one touch’s your bathing suit areas” we also use the correct names but it gives my kids idea on boundaries.
Don’t take the “they are curious” bs either .
I wouldn’t call it SA at that age but curious children who imitate what they see. Your nephew is either exposed to sA by an adult, porn, or watching his parents have sex in front of him. Be gentle with both. Teach him as well.
As a teacher, I have encountered this with my pre- k students. It is not something to ignore. I would take it seriously and seek psychological interference for the children as it can lead to devastating circumstances in the future. Especially if the boy was insistent or she was wanting it. Both red flags.
If this happened to my child, I would most definitely keep them separated when no adults are present and can only play with each other when parents are around. He probably was hurt this way before and that’s why he thinks it’s okay 
Your nephew has more than likely had the same happen to him! I would get her into some one to talk to! He also needs help! Your sister needs to figure out what is going on with him!!!
Kids are very curious about their parts around this age. You obviously Don’t want to dismiss the situation. I think both kids should be educated on their parts and what’s right and wrong. He needs to speak to someone he can trust and can make him feel safe, In case there’s anything he wants to share.
This needs reporting. A child that age shouldn’t even know how to do that.
That poor baby.
People moaning about him but he clearly needs help and intervention too
Children do go through stages of curiosity and they are only 5 and 6 this does not mean the child has been abused if they was older then yes I would question it.
I would speak to the children about it and say they cannot touch private areas as these are private.
Did your 5 year old use the word massaged? I would keep word simple and explain to them both
Ultimately speaking to his mother and trying to keep them apart is the best plan for right now. Also, maybe getting them together to talk or talking to them separately about how it’s not okay to touch each other’s private parts like that because 1 they are not old enough for that and 2 they are related.
I used to have a huge crush on my cousin (we never touched each other inappropriately) and one day my mom sat me down and told me that I can’t get married to him when I grow up because we’re related and our children could have issues due to this. I understood after that and moved on, but I would have never known if my mom didn’t have that talk with me (until later in life at least) I was like 5 or 6 at the time.
Kids are very curious and I don’t think they really understand what they’re doing.
He had to have seen it somewhere, or god forbid someone has touched him. Praying he just seen it on TV by accident or something. Def talk to her about no-no spots. Hopefully he’s talked to by his parents about it too.
As someone who is a victim of molestation. No kid that age should know how to do that. He either saw his parents do it, he learned it from somewhere, or he was harmed by someone already and he’s thinking it is normal. It is a big deal. Not something to ignore either. Cause it can continue if ignored. Need to talk to both of them and get them counseling.
Have we taught good touch had touch yet
I would obviously address it simply because you don’t know where exactly it came from and if someone has sexually abused him someone needs to know. Kids pick up on so much more than we think, weather it be from seeing it on tv, or it being talked about and them overhearing it. More than likely it is kids being curious, they both need to be sat down and talked to about it, in a calm and collected way without making them feel bad and if your unable to, find someone who is able to. My kids and I constantly talk about the importance of not allowing anyone to touch them there and if someone does they need to tell me asap. (To the point my daughter didn’t want her male doctor checking her down there during her doctors appt, but he respected her wishes and allowed her to be in control of her body)
Get BOTH children to a doctor and counseling. If he is touching her privates someone taught him about it. They both need checked for sexual abuse. Also make sure you are using proper terminology the vagina is the inside and the labia is the preside.
He could just have stumbled onto some inappropriate media and it is normal for kids to be somewhat curious about the other kind of genitals. I think the big question is if either child had been talked to previously about their penis/vagina, how they’re private areas etc, and good touch bad touch. If both children have an understanding of these concepts then I’d consider this a huge issue. If neither of the children knew then I’d consider this a lesson learned and a place to start educating them about their bodies. If one child knew these concepts and they other didn’t I would be having an in depth discussion to get to the root of the issue. So definitely figure out where he picked this behavior up and take their bodily knowledge into consideration and go from their with all the parents involved.
I would be talking to both children about appropriate behavior and keeping hands to yourself - your private parts are only for you etc
Educate both children.
Children get curious and tend to experiment it doesn’t mean something has happened because they are innocent and don’t understand that exploring bodies can have repercussions.
Communicate with your sister and get her to reinforce what you are saying to the children as well.
Supervised play for a while. Yes kids do ‘inappropriate’ things out of curiosity. But you absolutely cannot rule out that someone else has done it to him. That doesn’t mean it has happened. There’s definitely still a possibility of it being fully innocent at that age. Please don’t make either of them feel like they’re in trouble as that can cause issues in itself. Personally I’d only let them play supervised. Talk to each of them separately about keeping our hands to ourselves, especially for bathing suit areas. And ask simple, easy questions about if anyone else has touched them there. All calmly and nonjudgmental. They’re very young and have different parts. It does NOT mean either have been molested. But if it is going on, it needs to be addressed now.
I wouldn’t make her feel guilty about it. Kids get really curious at that age for whatever reason. That said, definitely can’t leave them to play alone as it will continue. The nephew absolutely needs a talk about keeping his hands to himself. Not saying scare him but he needs to know that touching privates is an adult thing and if there’s another incidence it’s going to create problems. It’s a super tricky situation and being a kid that was exposed to that you definitely want to be delicate in your approach with both your daughter and nephew. Best of luck 
Both children need to be evaluated for child abuse by an expert in the field. Yes normal to play doctor and explore but this is not normal behavior
You will have to ask her if this happened over or under her clouthes. Regardless of that answer your going to this boys house with your mate if you have one to disclose this to his parents you call a therapist for your child. It’s hard to stomache but we are adults here shes just had her 1st sexual experience there’s no control over what has happened only what happens now in the way that this is handled. She will need to know in the comming days that noone touches you down there not your parents friend neibhor uncle that is hers private parts and those are off limits to anyone but herself.
First off teach your children to use proper anatomy terms 
this is extremely important it’s the first thing a predator will notice that the child has been talked to and can communicate!! Instead of uncle ate my cookie you get the drift !! Never have cute names for your children anatomy!! Ever !!! This is very important second children are never to left alone !! This is an age where they become curious!! You should probably talk with a Coucler do not yell at the child !! Do not threaten child but reinforce that they are not in trouble but what to do if someone does anything or say anything inappropriate to an adult immediately teach them to scream if someone touches them and remind them that this is a good thing to yell !! As loud as you can !! Tell an adult immediately if it’s an adult or a child so not be embarrassed 
you did nothing wrong !! Your nephew may have been assaulted just be aware and have a very serious talk with mom !! Key no forts! never out of sight! doors are always Open! children never left alone with adults and or children always be aware of your children’s behavior I think they might have definitely done something inappropriate because of her reaction for sure as a parent always trust your instincts 
always remind your children when they take baths or you help with bathroom that those are their private places and no one is aloud to touch that’s your space only !!
Not all were molested or touched. Thier curious period. Early education look it up. Also on your note address both together and talk to them both. And discuss both ends and why not.
That little boy has been expose to sexual activities and you indeed should talk to his mother and get to know what’s going on.
Honestly, it was probably just curiosity. I was around the same age with a family friend and we were literally just curious. I remember our parents found us in a closet. And i remember we did it a couple of times after being caught. He wasnt any more guilty than i was, so please don’t put blame on your nephew. I know these are scary times, but I wouldnt necessarily jump to conclusions of anything more sinister than curiosity. For now, stay open with your daughter about her body and what is acceptable and not.
Keep them apart indefinitely.
Consult a therapist on how to proceed and have them talk to her.
Curiosity!! Honestly they are still young and just needs to be educated. Talk to them . Separately and then together
Don’t jump to the sexual abuse conclusion. Young kids are curious. Just teach your daughter about her private parts and good touches vs bad touches. Ask your sister to do the same with her son.
Kids at that age are extremely curious, especially about having different parts. I’m in no way shape or form saying it’s acceptable or appropriate, but it could have been simple curiosity. It could have also been a learned/taught behavior.
With that said, if I were you, the first thing I would be doing would be addressing it with his parents. Most parents, their instinct is to protect their child, and make excuses. I would sit down, you and your spouse with your daughter and your nephew with his parents and all talk about it together. Flat out ask your nephew why he did that and where he learned it. I would also have another one on one conversation with your daughter about good touch/bad touch and consent. She’s 5, so you’re going to have to repeat that conversation and give her reminders. Use proper terms and whatever you do, make sure both conversations are done calmly. As I said, a 5 and 6 year old aren’t going to fully understand what’s going on/what happened. It could be curiosity but it could also be imitation behavior from something your nephew has seen or had done to him.
Keep him away from your daughter!! Don’t bring him around and continue to talk to her about it and let her know she was so brave to tell you. Family or not keep him away.
Could be curiosity or he could have been molested himself.
Kids shouldn’t be alone together period. Doors open. And i wouldn’t be allowing him around , he’s seen or experienced something .
IMO a conversation needs to happen with both children separately and together. The one separately needs to be more explorative of feelings and educational about what is appropriate and inappropriate behaviors. The one together needs to be more matter of factly put that neither of them is to touch or massage anyone else’s private parts. They are just children now but this needs to be addressed because children need their parents to step in and address/correct inappropriate behavior because one day they will grow up.
keep him away from her talk to his parents
Do not have them together at all. You need to ask your daughter more in depth about this, and if anything has happened before. Your sister needs to talk to her son and find out if anything has happened to him. If not then she needs to find out why he thought it was appropriate and properly educate him.
Teach proper anatomical terms and they both need to be taught about their private’s being theirs and not for anyone else to touch.
You can then decide if you think a talk together would be appropriate but honestly I’d not be leaving them alone.
During my foster parenting classes they teach you if you tell a child that is inappropriate and the behavior continues then there is a problem but curiosity is normal.
Ok, calm down… they’re the same age . curious… he was wandering were her penis was … this is a teachable moment for both of these CHILDREN
Yes, kids are curious. But not to the point of touching another’s private areas unless it’s been done to them. As a survivor, I’d advise your sister to be vigilant about who her son is around unsupervised because something happened to him.
He’s 6. I’d have his mom talk to him. I seriously doubt he’s trying to sexually abuse someone at 6.
I had the same situation happen with my daughter and I am a victim of child abuse so of course my brain just kicked into high gear I talked to my husband about it and we made sure that whenever they were together they were not alone tried to make it not obvious .
They are 5 and 6. PLEASE EDUCATE THEM, that is your nephew not a random child. Get your sister fully on board to help this child.
Chances are this little boy I’d being abused… Talk to his mother about who is around him… Talk to him… Let him know he’s safe and won’t be hurt… He’s 6… That’s him projecting his feelings… He’s been taught that it’s okay and it needs to be addressed now… As for your daughter children councilors can be amazing. They can read into things that kids say or do that we’d think nothing of.
If they are together don’t leave them along, make sure you can see what they are doing at all times. Teach your daughter that no body is aloud to touch her vagina. Teach her what she needs to do if it happens.
He may just be curious as some children do or he may have been sexual assaulted as well. Keep a close on eye on everything
Do not let them be alone together any more
I was sexually abused for a while when I was younger. Around 7 years old…… I’m telling you from experience to talk to your sister about finding out who did what to him. Curiosity is normal ie- show me yours and I’ll show you mine. But to massage the vagina sounds like someone taught him that and that’s beyond curiosity. And to the lady above who said “he’s six. He wouldn’t sexually abuse someone at six…… he’s not sexually abusing anyone. He’s experienced it himself and doesn’t recognize it as inappropriate. Also- not to be ugly or assume…. But 9 out of 10 cases it’s a family member. Pay attention to how your sister responds, never underestimate anyone. Most of the time it’s the ones you least suspect.
For all of you saying "keep them separated " it’s a fine theory, but a knee jerk reaction isn’t going to help anyone.
Im with those saying talk to the 6 yr olds parents. Let both children know that they won’t be in trouble for telling the truth AND stick to it.
Sounds like natural curiosity. If you cut contact with no explanation to the kids, that can lead to a whole other lot of trust issue related problems down the line.
How long did you talk to her?? Did you make is seem like you wanted her to say something happened? Did you ask her point blank if her cousin touch her Virgina? Little kids are very impressionable and if you made it sound like that’s what you wanted her to say then that she would have said… Im not saying it didn’t happen but if it did then it was probably more of a curiosity thing because they are so little. I would let her know that under no circumstances is anyone allowed to touch or look at her privates unless it’s a doctor or you. Please do not think your 6 year old nephew is molesting her unless you know 
… If she tells you again then take action
If it were me I’d sit both kids down with your sister and explain good touch and bad touch. I’d ask him where he learned this from and depending on his answer and reaction I’d get him into therapy. Explain that you’re upset, but don’t show any anger because he is so young and I’d want him to always feel comfortable with me. Good luck!
When my son was 6 years old I caught his 6 year old friend sniffing his private parts. I talked to the boys mother. Come to find out, the kid walked in on his uncle having oral sex with his GF. They told the boy he was sniffing her privates to check for something, I can’t recall what. Sometimes, at that age, kids are mimicking something they saw or heard. Tred lightly and dont automatically think the worst. 6 years old is still a baby on both sides. Maybe something is going on in the other childs home that should be concerning.
This may be kids being kids. I would not Shane her and I would not make her feel as if what he did was wrong or right,. If you were right there and she was continuing to play she may have been okay at third with it. Kids are kids and that is not untypical. I would set clear firm boundaries and open communication for when your nephew comes to visit. No alone time, no covered bodies, periodic “hand checks” and the like. I would be more inclined that this was a kids being explorative thing than “sexual abuse”. My opinion, unless there is additional info that has not been divulged. As a victim of childhood sexual abuse that is just my opinion.
You should talk to them both separate and together about this kids get curious so you have to remain calm and explain it all
They teach sex Ed in school now also no boundary’s are taught,I mean everything!
Sounds like you’re 6-year-old nephew is being touched as well a 6-year-old is not going to do that unless it’s being done to them you need to get both children help
A 6 year old little boy DOESNT just do this out of no where. He’s way too young to even really understand what that is.
That means he either is being abused himself or he’s being exposed to/ seeing adults do inappropriate things in front of him.
You need to 100% keep your daughter away from him. Please listen to your daughter’s needs, and take her to see a therapist if she is showing signs of distress.
And he NEEDS to go to therapy or to talk to somebody himself. Someone is touching him or he is being allowed to see things that he shouldn’t.
I’m sure the other parent was shocked and my not even believe you…best thing to do is to not let them play alone together any more… without an adult supervision…
There’s a book for kids called my underpants rule, every parent should have it it’s excellent….& yes sit down with your sis and both babies have a talk with them & id be trying to find out where he learnt that from as well, hopfully not another adult or child could even be something he’s watched in a movie or seen online.
The world we live in now. So many twisted minds everyone assuming the boy was sexually assaulted just because they were both curious
I’m sorry that happened to your daughter. I’d bet a lot of money that the nephew has been touched himself or has physically seen it happen to someone close to him. Either way it needs to be addressed asap.
Was your sister concerned?
Talk to them both separately and figure out where a six year old is learning this…have your sister there too.
Educate both of them. Sit them down together and discuss what is VS is not okay. And teach them to come to yall as their mothers.
I don’t think there’s SA involved here. It sounds like curiosity to me
So my friend is going through something similar at the moment with SA. She called her child’s therapist. The therapist told her to call DCF. DCF is going to interview as well as having a child advocacy center interview. We read up a little bit about what normal childhood behavior is and what’s not. Some things are more common than others. You can look up on the website and check off what pertains to your own situation. I guess it’s more common for kids that are close in age and the more of an age gap, the more concerning it gets. I would reach out to someone to help you through this so that you can help her too.
I agree sit both of them down with your sister too and talk to them. It could’ve been something he may have seen himself or whatever (as disturbing as that is) but they both need to fully understand what is going on and why it’s inappropriate. If it continues then more drastic actions need to be taken for your daughters safety. Best wishes to you and your family 
Kids are naturally curious and they’re going to be curious with other kids they’re around. This doesn’t stem from “abuse”. Do not traumatize them for being curious. They don’t know. This is your opportunity to educate them.
Reassure her she did nothing wrong but that no one but you or her doctor should be any near that area.
Talk about ways for her to say NO and let her know she should always always come to you
This happens in daycare too! My daughter was 5ish when she told me a boy was touching her down there. I definitely talked with the caretaker and I taught her that is her area only she’s allowed to touch and if anyone besides mom and dad helps her at the potty it’s not okay and to speak with us. I haven’t heard anything again and she’s well aware of her pvt areas being for her to touch only.
When I was 5, my sister and another girl, and I were being watched by our babysitter, we were under the blankets laughing and just playing having a good time, when the baby sitter walked in, she wanted to know what we were doing, we told her and she didnt believe us and put us in the corner til we told her what we were doing (for the rest of the day) I genuinely had no idea what she even wanted us to say… I eventually realised she thought we were doing sexual things. So at 5 I had no idea what she meant, that little boy has seen things or had something done to him… I would talk to them both separately.
Farrah Smith well I also have young nieces and nephews
And a lot of the times they don’t understand what’s happening
Especially if the mother hasn’t had the ‘appropriate touching ‘ talk.
Education on this matter isn’t just a one time thing. Children need to be reminded constantly to do or not do something.
Couple of scenarios could be happening…
1.) the little boy could be being abused and doesn’t understand ,
it could happen …
2.)the parents haven’t had the appropriate touching conversation, which sounds like its definitely needed
3.) lack of supervision
Kids need to me monitored ALL THE TIME !!
I used to plan arts and crafts when my nephews came over. No playing house or boyfriend/ girlfriend games
4.) and this was common in my situation
We are a very loving family
Lots of hugs and kisses
My husband and I always have our hands on each other but I needed to teach my girls that we’re married , and inlove, and adults …
monkey see monkey DONT DO!
And it was a constant reminders that they couldn’t act like that in school or their friends. Only for married adults .
To them giving kisses and hugs was normal.
I say get together with you sister and be on the same page so when incidents happen you can both be form and nip it in the butt before it gets to be too much.
Mothers need to teach their girls to be strong and protective with themselves and it should start as early as 4 …
It could be simple curiosity which DOES happen around this age OR it could be something more sinister happening to/around your nephew. In my case, it was my cousin and he was being SA, but that doesn’t mean that’s guaranteed for your nephew’s case. The kids need to be talked to about boundries and body parts.
I’m sorry but making any child feel ashamed of what they did especially this little is not the answer. Definitely bring your concerns to the other parents and everyone needs to stay calm and explain why we cannot touch other people private areas family or not and what caused this it could of been curiosity it could of been more but making a child feel ashamed is likely going to cause them to shut down and not say a word.
Kid’s are curious, POINT BLANK PERIOD!!! I swear some people. For the record that age is NOT too young to understand and or be curious, some people must have not had other sex neighbors, family friends kids, opposite sex siblings ect ect ect and it shows… 
. I would just talk to them together about private areas be private, and then watch them, the end.
Obviously people don’t get how easy it is to see this stuff on huge platforms we let our children on everyday YouTube
You tube kids
Netflix
Even regular tv
I caught my daughter’s favorite show that I prescreened as appropriate in the beginning and now it has mutated into something absolutely awful in you tube kids none the less I immediately banned the show but doesn’t mean she wasn’t exposed and I had to have a serious conversation about what was appropriate and what wasn’t. Talk with your sister you know her home life better then internet strangers is her home life decent and good did you ever suspect abuse of any kind if not I’d have a serious conversation with her and have her talk to her child and then have a soft conversation with your daughter to scream bloody murder if anyone ever makes her feel that way again we also don’t allow boys in girl rooms here in my home as well as the opposite way around they can play where they are supervised by adults in common areas just to keep everyone safe and sound if definitely not leave them alone together after this but if it ever happens again then you know you have a issue on hand the other option is to have a camera in her room and other areas so you can actively see what is going on
Kids are curious they don’t know what they’re do but it’s definitely time for his mom to have a talk with him also…
Do not leave her unattended around him or anyone. That’s the hardest thing to do. Kids are exposed to do much and many parents do not protect their innocence for long.
He could of watched something, or unfortunately is being touched himself. It could be as simple as they are curious about their body BUT it needs to be handled.
Not shaming them but being firm with it not being okay or tolerated. They need to be educated with love and seriousness. Maybe even talking to a professional will help if you feel your daughter might experience trauma from it.
In my case and it will be briefly as I get triggered. I didn’t tell anyone until after 18 and I suffer from serious depression and suicide thoughts. Thank GOD and the Doctor that actually listened to me. I’m now on meds but a key factor for me would of been how mu parents reacted. I knew it was something frowned up and was ashamed to open up. Don’t give your little ones this feeling please!
First question where did she learn to talk in those words. He’s 6 so where did that boy learn that? So young momma I feel for you. Talk to the boys parents
I whole heartedly sympathise with you mumma. It would be putting my stomach in knots. I hope your sister comes to the table and you both can discuss where this behaviour has come from and both babies are protected and taught this was not ok at this age. Definitely no alone time play now.
But also, I most definitely understand the need to protect your young daughter. But I’m actually shocked at how many of you are treating this tiny 6 year old like a young man !!! He’s just a baby himself !!! There needs to some questions, finding out where this behaviour was learnt and best case he gets a little counciling and set boundaries about peoples body parts and what is and is not appropriate. Worse case his mother needs to do some serious damage control and protect that baby !!!
I hope you got some sleep mumma so you’re able to come at it from a fresh angle and I hope both babies are ok xox
Well my child was involved in something like this when she was 6 and we had police called on us and had to go through us being investigated and counseling and the counselor came to conclusion it was all innocent curiosity. So be-careful how it’s handled
If he’s doing it to her then someone’s doing it to him. Tell your sister. Don’t blame the children. Just explain what happened and say your concerned for him
Open discussion asap!
Kids are curious I constantly have to tell my 5 year old “hands above the blanket. Please stop touching yourself, you’re too young to do that.” She said “ it feels good” when I asked her why she does it. She says no one has touched her and I believe her.
Kids are curious about their bodies.
I think maybe these young kids are curious but def communicate/get to the bottom of this. Embarassing them for being curious could be devastating too, talk to your sis. Better to embarass you/her than them. Prob dont leave them alone again either
My concern would be finding out who’s doing it to the 6yr old …
You need to have your doctor checked my a doctor and have her talk to a Children’s psychologist to find out what’s going on. Also the boy needs to be checked out by doctors as well because most likely something is happening to him. Young children who do that to other children are victims of sexual assault themselves.
Coming from a law enforcement officer who predominately works these types of cases, if you talked to your sister and she wasn’t receptive to having a boundary setting conversation and possibly even counseling for both kiddos, you can file a report with your local agency. What they’ll do is forensically interview the children and then based on that they can move forward with like court mandated counseling. I don’t suggest that option tho unless your sister is completely denying the allegations and not receptive to counseling etc. but I would make sure that you’re having very informative talks with your kiddo and making sure she knows that no one is allowed to touch her there without her permission and that she can always come to you when she feels scared etc.
We live in a world where kids can see anything. You have kids on social media platforms that are way to young and listening to people who say crazy things. Yourself and your sister have to talk to them but word it where they can understand. Separation needs to happen though until yourself and your sister talk to them.
I would also add, which it sounds like you were doing, but for anyone else reading this thread, that it is of the utmost importance to teach your kiddos the anatomically correct name for their private parts. This can help open lines of communication in the future should the unthinkable happen.
I’m sorry but no 6 year old knows that doing that intentionally is a form of sexual abuse. I do not believe the young boy has any idea what that even means. He could have been abused or molested himself. Talk to sister ASAP and emphasize seriousness.
I have 4 kids, 3 girls and a boy and I have multiple cousins, nieces and nephews and this has NEVER happened. There is something weird about this. My kids have never played like this and we have had talks about there bodies etc. I am not sure about everyone saying it’s normal because it’s not
When my niece and neighbors daughter was over, I had a discussion about what had happened with my sister and neighbor. I told them I would much rather have them know about it than brush it off irresponsibly. Kids are curious about body parts starting when they first accidentally realize a new tough that “feels weird”. No need to bash them or else they will wind up rebelling and finding out from elsewhere or from someone else. Us adults have to pay close attention to our kids when stuff like this happens and respond better.
As someone who has worked in child welfare and mental health with survivors I want you to know that it is absolutely normal between the ages of three and six for children to explore their body parts by themselves or with each other in the same age group if a child is more than five years older than the other child then there is serious cause for concern.
However using the term massage would lead me to ask a child to demonstrate that on a doll to get a clearer picture of what is actually going on. I definitely would have a conversation with the other child’s parent because something may be going on with that child that they are not aware of. I do not advocate for telling other adults who are not directly involved such as neighbors Etc due to the fact that many perpetrators AR known trusted individuals and often in the perpetrators mind hearing that something may have been done to a child enforces the fact that oh well it’s already been done so I can too.
I taught my children at a very young age what their parts were and the names for their parts and I informed them that it was okay for them to touch their parts but only they could touch their parts and that they could not touch anybody else’s parts and nobody else could touch their parts until they were much older. I also told them that it didn’t matter if they like boys or if they like girls or if they like the boys that wanted to be girls or girls that wanted to be boys as long as whoever they like was kind to them and that anything that is supposed to be secret is not healthy and to never ever keep secrets from Mom because Mom will never be mad at them But never ever ever shame them or let anybody else shame them.
I wish you well
Too young to be curious behaviors. I encourage you to see where the student learned it
I agree with a lot of these answers. Kids are curious but I wouldn’t necessarily push the fact that someone else may be sexually abusing your sister son 
you and your sister need to sit down with BOTH kids and have that conversation of touching and how it’s not okay to do.