A lot of medications cause loss of appetite. Her perscription should come with a list of side effects.
If she's interested in losing weight, you could help her do some research on weight loss. You'll easily find information on the web that clearly states that the body will stop burning calories and will hold on to the fat if she stops eating b/c it'll think she's starving. This is a survival technique that the body naturally does. You can find healthy recipes on the web for her favorite foods and calorie calculators that will show her how much she needs. Maybe it'll help her continue to lose the weight in a healthy way.
It's motivational to me to know that I have to eat in order to lose weight. I plan my meals to make sure they're as healthy as possible and calculate the calories, fat, etc. I also replaced my ingredients and foods with all all natural, organic versions. I try to eat as much healthy stuff as possible but I don't deny myself things like pizza, pasta, and burgers. I prepare it myself so that it uses the most natural products and I can't tell a difference in the taste. I exercise and lift weights about 30min. to an hour everyday and with my new eating habits, I've gone from 140 to 128 since mid-March. Her body will feel so much better and she'll actually continue the weight loss, whereas not eating will soon stop working as well.
This is hard advise, but please hear me out. Although for a lot of people anorexia is about control, but for me it was almost an addiction. It started with rapid weight loss, when I didn't have time to go grocery shopping one week and was only eating brown rice (the only thing in the house) for 5 or 6 days. I loved the results (I AM NOT RECOMMENDING THIS!!!) it felt good for my clothes to be loose. I liked the way it felt to have an empty belly. I lost the desire to eat. Lucky for me I had people around me who dialed the peer pressure way up and pulled me out of what could have been disasterous. (I dropped from 180 to 120 in very little time) As somebody who has been through this I would say something to somebody for your sister sake. Not eating is something that can be addictive (at least for me it was). The drug might have curbed her appetite, but I doubt it did that much. Remeber it is better to make her mad now, that to lose her.