My younger brother bullies me really badly...?

So my little brother is 12 and I hate him to death. He's your typical jock/jerk/player/gangster:trouble maker etc. He has a new gf every week, he gets kicked outta school all the time. He has a wicked mouth on him and he says really bad things. He talks back at my mom like she's his slave and all she does is bow down to him because she doesn't wanna fight. He used to be a really good kid and now he's as rotten as hell. (I'm 15 btw forgot to mention that) my brother always tells me I'm fat or makes fun of me or tells me I'm ugly or retarded and I'm not I only weigh 102lbs (no I am not anorexic!!!!) and I'm fairly good looking ( not to brag I'm just saying) it's not always the things he says but the way he says them. It's hard to explain but I always feel so powerless against him. Like just a while ago I was practicing my dance and he was telling me how I sucked and when I was gonna turn that horrible music off and my mom was saying the sam things. Then he was teasing me all the way up the stairs. I really want to grab him and beat him cuz I'm pretty strong but he's too fast and if I did beat him he'd turn around and grab me and smash my face into the floor and step on my neck and stomach until I was bleeding and bruising. He doesn't care that I'm a girl. He's done it before. And even when were on good terms he constantly makes fun of me or he gets pissy when ibtell him something or show him something he tells me I'm really weird like everything I do is weirs and retarded. He constantly tells me I'm gonna be his maid someday and he'll be swimming in money. That's another thing. He's EXTREMELY arrogant and selfish. If there was a homeless baby at his feet begging for a crumb of his cookie he would shove the whole thing in his mouth and taunt the kid. I think part of the arrogance comes from how everyone at school loves him he constable has girls flock to his sides and guys always hovering around him. He knows I'm not as popular so he constantly makes fun of my friends and it makes me feel bad cuz I love my friends and I try so hard to defend them but he keeps digging and digging at them. he's also arrogant because he has everything he wants. He has an 82" flat screen tv in his room, an iPod touch, a phone, a game station a bunch Of games, a HUGE room it's literally the size of a kitchen or two. He has a closet half the size of his bedroom. You could put a dining room table in his closet it's so big. That just how my parents designed our house. Because I wanted a bathroom in my room I got a teeny tiny room and a little tv. So my step dad favors my brother. When I'm explaining what he does to me it doesn't seem that bad but if u could see him in action u would be so disgusted ud wanna bash his face in. My mom does absolutely nothing he always gets away with everything. Idk what to do anymore but everyday he makes me cry. And he always tells me je can make me cry so I'd better watch my mouth so I try to hide crying. I hate him so much I wish karma would just get him already....