LM, never suffered, but the missus did for eight years, and also went down the GP doesn't really know much about it route. It's also worth noting that this is a much wider issue then most recognise. With the excption of a few "well known people" the vast majority of sufferers, suffer without even the knowledge of their' partners. My guess is that a good number of readers here, have a partner who suffers, but don't know about it.
Everyone's different, but the "triggers" behind it, are often based around some of the following, with all of the usual influences from early life.
Self Image, diet, esteem and life goals. Body shape, often as a result of having children.
Despite watching the missus go through this, and trying to be as supportive as possible, like most, I still find it difficult to comprehend, but no more then I used to watch Soldiers make themselves sick at the end of a night out.
My only advice is to try and talk to your sister about it and be completely open with her - the "secrecy" of the subject is the biggest barrier. Once you are aware in real-time, you can help. There are any number of drugs, many of which work, by suppressing appetite. Personally, they weren't right for the missus, but may work for some. Important to realise that some sufferers will carry out other forms of self-harm, if they feel under pressure to just stop. Possibly the best aid to recovery is to have a GP who is very experienced on the condition.
Good luck to you and your sister. It's a bit of a taboo subject, which doesn't do awareness any good either.
Is triggered by insecurities about body shape etc but there's usually some underlying depression/anxiety that means the sufferer finds it harder than normal to cope with those insecurities. Sort that out and you're half the way there to sorting it all. Professional therapy is the ideal if she'll ask for that help - everyone thinks they can do it on their own but the majority do need that extra support.
Good luck to you both. It is a hard obstacle to overcome but do-able with the right support. Wish you both all the best