What is treatment for anorexia like?

My sister has anorexia. She's 17. When she was 12, she was really skinny like my mom. She started putting on weight though and got to the point where she was 150 lbs, about 10 lbs overweight for height and age. She got a few comments that hurt her, especially from my mom (although I don't think she meant to hurt her). Finally, at the beginning of March or end of February, she changed... She was more friendly, doing a bit better in school, and losing weight. But she was losing weight wayyyy fast. It's only been a few months, and she now weighs 102. She has gone from overweight to underweight so fast! A few days ago, I found a "weight loss journal" in her room that finally proved my theory that she was anorexic, with pictures of exceptionally thin girls as inspiration, journal entries EVERY DAY that talked about what she ate and the calories and how many hours she exercised, etc. So my parents are trying to figure out what to do for her now that we know this is really, really serious. Does she need outpatient or inpatient treatment? What is the treatment like? What can we do to help her as a family?