My sister is bulimic and has been for a while now. SHe went to treatment, but doesn't really be open and seems to get nothin out of it. About a week ago she cut for the first time, but she doesn't know that I know this. I feel like no matter what she won't accept the help provided.
It may sound bad but you can report this to your parents and to your sister's doctor and you can write her a simple letter saying that you love her and want to grow old with her so that you have someone to talk to when youre 88 and that you would like to help her and she should tell you how to do it...but apart from that be there for yourself. Take good care of yourself.