The situation is terrible 
. I have been caring for my 14-year-old niece since her father died a year ago. She is profoundly troubled, with recent instances involving self-harm, drug use, school suspensions, and runaway attempts. Despite treatment and other efforts, her behavior remains, causing stress in our family. No one else in the family will step in, and prior guardians relinquish custody owing to identical concerns. Her stepmother seeks custody but faces significant legal difficulties. Can a non-biological guardian abandon custody? Would her absent mother feel forced to take her in? 
Perhaps you should try to support the kid as she needs good people, she seems troubled and could benefit from positive influences in her life. I understand she might be challenging, but it’s the right thing to do.
I suggest you visit a therapy.
Teenagers may be difficult to manage and a child who has been through what you have mentioned requires both love and support.
She’s already dealing with significant mental health challenges, and abandoning her will only make things more difficult for her.