throuple

My step child lives with her mom( no court order just what they decided) My fiance gets to see her whenever he wants but gets to have her every other weekend. Recently my step childs mother came out as bi. Thing that is weird is the guy she's with cheated on her with their room mate. So instead of breaking up with the guy they just add the girl he cheated on her with to the relationship. Point of this post is my step child feels very uncomfortable living there now. She said her mom makes.her slave around the house. She has a 1 yr old brother and basically she( an 11yr old girl) is raising her brother because her mom doesn't wanna be bothered when she gets home from work. She even feeds him his breakfast and lunch. It's just outrageous. She's 11. She told her father she wants to move in with us. So we don't know what to do because we are in no situation where she can live with us which is why she is with her mom.(we live with my parents in a tiny apartment that we aren't even allowed to be at) She did at one point live with us from when she was 7 to 10. Her father is thinking of asking her grandmother to take her until we get everything situated. What would you do in this situation?