The first thing I would suggest doing is getting a counselor. Very good first step.

As for the sister, tell her the facts. Tell her exactly what was said and how terrified her daughter is by it. Make your sister aware of it.

To help your mom out, reassure her that you love her and will support her each step of the way. You could maybe even go to some of her counseling sessions with her, if they'd allow it, just for support.

Definitely tell your dad how you're feeling, what his behavior has done to you, and how totally scarred you are. Don't ignore him. That may make things worse. I know it would for me, anyways. Be on guard when you talk to him, though. Just say the facts, don't use blaming or namecalling or anything like that. If he resorts to namecalling or saying abusive things when you confront him DO NOT react to it. I know that's easier said than done, but this is very important. If he starts the abusive bit, don't feed into it. Ignore the attacks, but don't ignore him. He may try to get you off topic but keep refocusing back to the original topic.

Good luck. I hope this helps some.