Thanks fellow miscreants. Sadly she's on the Gold Coast so I'm pretty much an ear at this point, even more so since she made me promise not to tell our dad and her mother. It would've been nice if the black dog had skipped past one of the family at least.
I have what I guess you'd call an ex step daughter from a previous relationship whose life I'm still in somewhat as the ex and I have always been on good terms.
She had a serious bout of cutting around age 15-16 including an incident where she passed out from it in the bathroom and went through the shower door.
She had always been, and is, tall, skinny, smart, pretty and highly socially functional; this bullshit really can happen to anyone. Expectations is a key factor for teenagers I think. One of the things we did was tell her to stop even worrying about uni; go when you're 21 because the STAT test is way easier than stressing too much about year 12 exams. Not saying that's great advice for everyone but I think it was right in this case.
Her mum did what you're meant to do, shopped around for therapists until she found one she liked. She's 18 now and seems to be in a good place since high school finished, working, travelling and generally breaking hearts all over the place.
Good luck, I hope you can keep an open dialogue with your sister and help her through this.