You can check out your area for some Alateen meetings, and then suggest to your niece to try 3 or 4 of them, even volunteer to take her.
There she will find her 'peers' that are going through what she is going through. Explain to her that it is not something she did or didn't do, that they are 'sick' and part of the sickness is to tell them that they are not (denial).
Be there for her as you have been. She has one stable person in her life that way, and she can and will see that recovery is possible.
I really think Alateen would be a big help to her.
J M H O
Love and hugs,