Can a 16 year old child choose to live with a relative other than their parents?

My sister is refusing to allow me and the rest of my family access to her children because of her abusive husband. The child in question is not biologically his but they managed to place his name on her birth certificate anyway. These parents use this child as a babysitter, cook and maid as they have used her since she was at least 9 years old. She wants to come and live with me where I can support her and keep her in the same school. There is obviously bad blood between the family members because of her choice for a spouse. Her oldest child who recently turned 18 was taken from her when I believe he was 3 because of this man and given to his biological father. What can I do to legally get her out of their care? These people are very sneeky with the law and find ways around it.