You seem to be bearing a lot of weight on your shoulders. Your sister definitely needs to communicate with a therapist and/or a psychiatrist, and it would probably help you and your Mom, too.
If you can't get her to a therapist or a psychiatrist, maybe she could post here. She'll find that people here are understanding and supportive.

I don't know much about internet counseling, though I've heard of it. Does anyone else know if it's reliable?