Wow,I can completly relate to this post. There's not too much you can do really. Personally what I do is sneak my little brother and I out of the house and I take him over to the store to get something to drink, or something. and when they try to drag you into it, tell them they need to stop acting like children, and take care of their priorities in life. Do NOT let them drag your sister into thid though, that will mess her up. Badly. if they continue this, or ANY violence is going on, I would STRONGLY consider telling some authorative figure, because this could get pretty ugly, and perhaps dangerous. Before you decide not to tell anyone about this, consider that your sister might be in danger too.
Please be safe,
and you are not a ad person. you sound like a wonderful person, who deeply cares about their little sister.

You always have friends here.