Hi, and I'm sorry for your difficult situation. I think you're a great sister to be worried (and, I think, with VERY good cause) about your little sister. I hate to have to suggest it, but it seems to me that you're going to have to disclose this to child protective services (or whatever equivalent type of authority you have where you live). This is a very dangerous situation she is in.
Is there any possibility she could live with you and your boyfriend? If not, your sister would probably be much better off in a loving foster home (where you could continue to maintain contact with her). I know that sounds very scary, but you could continue to be a stabilizing presence in her life.
Hugs,