Please help me out here...?

I don't know what to do anymore. To say that I live in a dysfunctional family would be wrong. I don't live in a family. Things have never been good since I moved in with my Dad and Stepmom a year ago, but lately things have been getting worse and worse up until last night when my Stepmom was highly intoxicated (she is both an alcoholic and a prescription drug addict) and started a VERY large fight with my sister. My parents have been horrible towards her since she decided she wanted to move back home, and now she is leaving on Thursday. The thing is, she is being told that if she even says one word to them unless spoken to she is going to be sent to the New Mexico Military Institute, and out into the system which will supposedly make it extrmely hard for my Mom to get her home. The man I have the misfortune of calling my Dad has told her on more then one occasion that he will want nothing more to do with her once she leaves and he will never speak to her again. He has also told her that she is a worthless piece of sh*t over the phone, and called her a lying bi*ch and has told me many times that she is worthless, and that she will end up pregnant by the time she is 16. But back to the fight, my sister had her boyfriends hat thrown at her and it hit her square in the face (by the stepmom. Same for all the following) had her blackberry smashed to pieces, had her friends iPod taken from her and threatened to smash that too (and we are pretty sure she did because we overheard her last night talking to someone saying she didn't have the money to replace "it") and after the phone was smashed, the screen was broken up and had a bunch of sharp points to it. It was thrown at her and hit her in the head. There was no mark, but that was because she was just barely able to duck in time. This all happened before I got home, but when I did, the first thing that my stepmom said to me (I think, I couldn't really understand her) was We need to talk. I'm not drunk...and went on from there. This kind of stuff goes on all the time here. My sister even has a scar on her face from where my dad hit her and made her bleed (not a very big one, but it's there). Now my sister is going back to Florida and I am so lost. I don't know what to do because once she leaves, I will become the scapegoat for their rage. I can't go back to Florida because of two reasons, the main reason being I will not be able to get my High School diploma. I don't know what to do because if I go to someone about this, then I will get put into foster care and I don't want to have to go through that. But if I stay in this house then I don't know what will happen to me. Apart from all this, my parents have loads of guns in their closet, and after all that happened last night, I was terrified that the Stepmom might actually try and either shoot her, me, or kill herself. I am terrified. Please someone tell me what to do. I know that their are thousands of other problems in the world, I know that their are innocent people dying because of human trafficking, drug cartel, hunger, etc, but I just can't deal with this anymore. Please someone tell me what to do. I am desperate.