i used to cut, and so i know and understand what you're talking about. first of all... thank you. thank you for being such an understanding sibling. thank you for wanting her to get better. thank you for trying to help. i guarantee you, this will help her a lot.
most of what helped me to get over cutting is learning how to get the pain out with ways which are not physical. i know this totally sounds cheesy, but venting really works. allowing her to have a place to go is really the key thing. and whether she takes advantage of that - well, that's where personal growth comes from.
a journal, a pet, a sibling, anything will do. thank you for being open to her. keep letting her know that you're there. even tho she may push you away, she really will be happy to kno sumbody luvs her.
and tell your parents. get her to a doctor who she's comfortable with. medicine does help. take it from one who knows.
thank you, good luck, and best wishes
*~*jEnNi*~*