It sounds like a no-brainer to me. Dad doesn't want her, mum doesn't want her, she doesn't want to be with either of them, she wants to be with you, you want her to be with you. If it's officially acknowledged that it's OK for someone to move in with a friend/parent for a few days following a row etc with a partner then I can't see why she can't move in with you. The dangerous time is just after the move; a single-direction move must be safer than a temporary move to another home and back again.