I don't know what do do about my stepmom?

She (my stepmom, Crystal) has been with my dad since I was 6 years old, for about 8 years now. She is bi-polar so she is unpredictable, I can't stand being around her right now. She has always been the authority around the house, my dad barely has anything to say about what goes on around here. My dad used to be fine financially but now there are two babies (2 years, and 9 months) plus me and my stepbrother. She has bought so much crap since they got together (WITH HIS MONEY), I consider her to be a hoarder. We even almost lost the house a few years ago because she was freaking retarded and didn't make the house payments!! I am actually pretty well behaved I think... I get good grades in school, I hang out with good people and I don't date or anything like that so I am not very hard to manage. She constantly berates me, and grounds me for stupid reasons like getting on the computer before doing homework, or ARGUING with her. It is so SOO SO SO hard not to argue with her. The effort it takes to keep my mouth shut during arguments is so severe, that when I do shut up I have to go into my room and cause physical damage to my pillow while imagining it is her, before I can calm down (I would never cause harm to my stepmom, that disrespect would make me sick). But it is so hard not to argue with her because its like she eggs it on, just DARING me to say something. The tone in her voice with me almost all the time is rude, like she is talking to a dog, and it makes me angry enough to argue. I don't even think she realizes how she sounds when she tells me to do something. Crystal is the only person who hands out my punishments, the only one who tells me to clean, the only one who tells me to do ANYTHING. She has such control over the house that I don't even consider my dad as an authority figure much anymore. My dad even gave her the permission to slap me if I say something to offend her, is what she told me, in which I became so angry that I moved over to my grandma's house for now. She has such stupid rules, which is one of the reasons I get berated so often. Like how she says that I should clean without her telling me what to do. I am such a disorganized person, I NEED someone to direct me on what I need to do. If she would simply tell me to clean the kitchen or something, it would make life easier for me, but she insists that I should, "Do what needs to be done!" LIKE WHAT?! I don't know if I will do too much or too little. She tries to make me babysit a lot and I make it as difficult as possible for her because if I do too well, she will make me babysit more and more, realizing what a cheap reliable source I am. She already makes my 8 year old stepbrother babysit alot.. I feel kinda sorry for him. I just don't like babysitting, because small kids just irritate me too easily. I don't have the patience for that, and that is what she doesn't understand. Crystal just doesn't know anything about me, what I like and dislike, I don't think she even cares. She thinks she is always right, she just won't accept to being wrong, which annoys me greatly. I remember being so much happier when my parents were still married, the house was a clean place and me and my half brother were treated well (we live in different places now). Now its just a nightmare. I am so exasperated and I feel like I am alone in the entire situation, and that nothing can be done. What should I do?