I think my sister is anorexic?

She's fifteen years old, 5'4" or 5'5", and weighs somewhere in the 120s. She is active; she goes on runs, and until recently she took martial arts. However, she thinks she's fat and eats about 1200 calories a day. She beats herself up for doing things like drinking a shake that had yogurt, dates, and a banana in it for breakfast. Once she asked me about the 700-calorie diet; after I sent her a lengthy e-mail explaining what a stupid idea it was, she seems to have given up on it, but she still eats way less than she should. Mom has a rule that she can't go below 118 pounds. She thinks she'll be happy with herself once she reaches that point, but I seriously doubt it. I told her curvy women are considered attractive these days, but she doesn't care. She doesn't even feel healthier now that she's lost all that weight, but she's more concerned with being thin.

She has been concerned about exercising and eating right for a long time, but it only became worrying in recent months. Since this summer she has been seeing a therapist for anxiety and self-esteem problems, but it doesn't seem to be helping much.

She is aware that she has a problem. She reads books on eating disorders, and once she said she was afraid her diet would hurt her muscles. She hasn't started eating more, though.

My parents know about her diet, but don't seem to think 1,200 calories is too little.

Any advice?