My boyfriend and I don’t want my sister to watch our son due to comments she’s made. Advice?

My sister always asks to watch my baby, or pick him up from daycare. I work every other weekend and my boyfriend does occasionally, and when that happens my boyfriends sister watches our son. We keep it quiet so my sister doesn’t find out so we can minimize the times she asks to watch him, but eventually she’ll find out so I need advice on how to handle this. To make it clear, we decided she wouldn’t watch our son before he was even born due to how she treated her own child.

A little back story: My sister and I are close, but she has said and done some things in the past that makes my boyfriend, and honestly myself weary of her.

We have witnessed her yell at her then 1 year old son to “shut the f**k up” when he was crying. She made a comment about my son that if she had to deal with him crying the way he does (he’s colic) that she would’ve smothered him if it were her son. She’s also made a comment in the past when her son was younger about how when her son was crying she thought about putting a pillow over his head…

She also makes weird comments about my sons genitals, how it’s long. I put diaper rash cream on him and she thought it was funny to say “oh yeah mom touch my balls” and it made me extremely uncomfortable that she would say that and basically sexualize something as simple as putting ointment on my baby’s butt.

I know she wouldn’t hurt my baby, but we also aren’t comfortable with her watching him by herself because of the way she’s treated her own son, and the things she’s said and done. We feel like if she can treat her own son badly, then what would stop her from treating ours the same or worse, especially if he’s crying and she can’t handle it? She acts loving and caring towards my son whenever she sees him, she just says weird things.

Are we over reacting? She has a dark/shitty sense of humor, and the comments shes made were in a “joking” manner. She’s said them around other people and nobody else says anything and it doesn’t seem to bother them so that’s why it’s hard for me to “be the bad guy” because I’ll be the first person to call her out for what she’s said.

I am having severe anxiety about the possible turmoil it will cause, I’m really lost on how to approach this because right now I just keep making up excuses but that will only take me so far. I’m dreading the day that I have to approach it, I hate confrontation and conflict.

I really need some advice on how to tell her she can’t watch him, or if I should just continue to beat around the bush and make up excuses. What would be a semi nice way of telling her? Or are we are being ridiculous? I’ve talked to people about this and they brushed me off and said she’d never hurt him. Please help!