I have a strange situation that is very tough for me. SO if anyone has any advise I would greatly appreciate it. This might be a little long. sorry. Bear with me please.

I have a 12 1/2 year old stepdaughter that I have raised since she was 18 months old. Her mother is not really in her life at all even though she is allowed visitation. Her father and I separated about 2 years ago. And the divorce was finalized earlier this year. Her mother has mental illness and drug issues as well as her father. If you know about mental illness, her father is schizoaffective with bipolar tendencies. I also have 2 biological children with the same man. When we separated she went and lived with him. After a few months, he had a moment of clarity and realized that he could not take care of her. He asked me to take her, so I did. We had a power of attorney drawn up. After a while I had to get a protection order against him. Both of these legal documents have expired.
The kids see him every other weekend. She wants to live with her dad and he wants her back. I am having alot of difficulties with her lately. She is incredibly mean and rude to her 10 year old brother who has Aspergers. She also constantly is saying things about him being a pervert and accusing him of doing things to their five year old sister that is not true. This is all stuff that her father says. I am worried that her rude mean behavior is affecting her brother and sister. She causes alot of fights around the house. I have agreed to let her stay with her dad during the summer, even though I know it wont be mentally or emotionally safe for her. This is what she wants and I really have no legal control. If he doesn't want to let her come back after summer, I cant make him. I am feeling very guilty, because I know me and my two children will be happier and peaceful without her. I also have a man that I have been dating for a year. The kids have never met him and I just recently told them about him. We are wanting to move in together by next year. Both of my Bio children are happy and excited. I am worried that she is going to cause a huge strain on mine and his relationship. I love her as my own child and want to protect her. I have been the only stable parent in her life. I'm not sure what to do. I sometimes really think I should just let her go back to her dad. It's what she wants and he wants and I have no rights. It would also probably make my and my childrens lives better. I dont know what to do.