Involuntary vomiting and bulimia?

My younger sister has had issues with bulimia. She is trying to get help with her problem, and she has stopped making herself vomit. The problem is, she now vomits involuntary. Every time she eats she throws up, without forcing herself. Our mom gets almost angry with her when she vomits, but she is no longer making herself do it, it just happens. She, and my sister's step father all just say "well you need to quit unless you want to end up dying". THe doctor told her is was all psychological, which in a way it might be, but what does she do now? She wants to get better but can't. SHe doesn't even have a menstrual cycle anymore because her body is so out of whack. She is about 5'2" and weighs 85 lbs. I want to help her but don't know how, and it seems no one else does either. Our mom gets angry and the doctor seems to think it is no big deal. She is only 15 years old, and the rest of her life is a long time to deal with this problem.