S.B.
It seems as though this girl obviously has problems and sending her back to a mother that you say doesn't want her won't help her any.
Child supprt should be the least of your worries.
I would contact the mental health department or the department of juvenille services to see what they can do to help you with intervening before her behavior gets more out of control.
I know it's gone on a long time and your family is tired, but one more person not wanting her will only make her anger and acting out worse.
That's just my opinion.
I'm not in your shoes and I don't have any answers, but surely there is help beyond giving her to a mother who has already had her visitation rights taken away from her. That would be like pouring gasoline onto a fire.
Kids really can get themselves turned around. It takes lots of work and patience though.
If you feel you can't give her what she needs or she is harming the rest of your family, look into other options.
Seek help for her. Not just another place for her to go.
I wish you the best.