Why is my mom so mean to me?

I mean my moms nice an all but too me not munch were not the same and she calls me name like stupid,idiot. Like today she slapped me! I have this thing when I drink a pill I choke she told me if I don't swallow it shell hit me I told her I couldn't and then she slapped me. She always treats my step brother(different dad) good. She always calls him angel or the love of his life. I always clean around the house while my brother is just watching tv my brother even calls her names and she won't care or dosnt ground him. Sometimes I think that she doesn't like me cuz of my dad. My dad use to hit her. And this other day that she got me got me food from burger king and they got my order wrong I told her and all she said was oh well my grandma got up and started cooking food for me so I would eat something. I even call her mom cuz she acts like one to me. What do I do?