Hi V., I know this is hard for you, and even harder to hear is that until she is ready to be done with alcohol and drugs, or whatever her addiction is, she won't stop. My husband has been clean and sober for 12 years. I have known him for 11. I can only say that after this many years, he is finally blossoming into someone new, and the damage is melting away very slowly. He was about your daughter's age, too, when he started using. I can't imagine him ever being that way, but he was- and his parents saw it all. One of the things that really did help him, from what they have told me, is counseling. You may have to try out a few different ones, but counseling is huge. Also, kids suffering from some kind of depression or chemical imbalance will self medicate because they know they feel good when they are using, and they are constantly after that feeling of goodness. Being lonely can also make it worse. Your step daughter, among all other things, is at home, in her room, on independant study. There are no friends, no outings, no friday night football games to look forward too...granted she has not earned your trust, but still. The loneliness can overcome a girl in no time flat.
I would say to not put her in a teen recovery group unless it is all girls and you can attend with her- boys are a distraction and addicts very often find comfort with one another, leading to unhealthy relationships because they themselves are unhealthy. Other girls, though, can empower one another and encourage one another. She might need a light medication, and I would suggest again to get her a female counselor who understands not only addiction, but the road to recovery.
Be there for her, be there with her, but don't enable her. Eventually she will be able to walk on her own- the deal is she needs to be convinced and truly believe that it is time to change in her own mind.
I hope this helps.
Good luck. Prayers are with you.
-E.