I want to get third party visitation or full custody of my niece

Hey mammas, i dont post anywhere really but could really use some advice. Long and sad story, i love my nieces and nephews to absolute pieces, they are like my kids to me, i lived with my sister and her husband for a couple of years after my divorce, after my sisters husband passed away she said and did hurtful things to me and my daughter, i ended up moving a way, almost a year later my sister got arrested for abusing her children, all five of her kids. If i had even an ounce of a thought that she would ever hurt those babies in anyway, i never would have left them alone with her, and that will haunt me for the rest of my life. They lived states away, and at that time i did not have the money or housing to get custody of them. They were placed in foster care and eventually released into the custody of family members after i has contacted said family members begging them to help the kids. Four of my nieces and nephew’s ended up with there aunt on there dads side. She has been amazing for them, she will be adopting them soon, she has been more then generous and understanding about my relationship with kid’s, and lets us visit and makes plans with us. My oldest niece got put into the custody of her biological dad, i thought this would be good, and he promised to let everyone on all sides of the family see her. Since then he has made minable to no effort and goes out of his way to ignore me, everyone in my family, and everyone in her adopted dads side of the family. He wont let her brother’s and sisters see her, her grandparents, her aunts and uncle’s. He has gone out of his way to isolate her from everybody in her family. His mom has admited a couple of weeks ago that they take her to a hypnotherapist to “re call” memories, ive looked up and talked to everyone i know who knows some things and they have all said that hypnotherapy to recall memories is not and actual practiced form of medicine, and it can also implant false memories in her mind faster then it would give her back memories she doesnt have. She also admitted to giving my niece horse tranquilizers to “help” with memory recalls. My heart is torn up, my kids miss there cousin so much, my nieces and nephews miss there sister so much. My sister understands that she should never have custody of her kids, and has said that she is willing to sign her parental rights over to me regarding her eldest daughter. My question is if anybody knows the possibility of me getting third party visitation, or full custody of my niece? I dont know where to start. I just want to see her. We all want to see her. She has so much love and support that she doesnt even know about, she deserves to have a relationship with her brothers and sisters, they have been through so much and i just want her to know that she has her family, ALL of her family around. (There is a restraining order for all of the kids against my sister because of what she did to them. All family does not include her unless that were my nieces wish.)