I hate my step brother.?

So, my dad married this woman and now we have to move into their house. My step mom has a son named Jason, the starting running back and the person who has to play defense against the other team of our school. I personally don't like football and think it's pretty stupid so I don't follow. Anyways he's a total jerk to me at school and always picks on me and cracks jokes about me to which all the other "popular" kids laugh. They call me a freak because me and my friends are scene. Jason tutors my girlfriend and I think she may like Jason. Jason is really big and buff because he's a muscle head and spends all his time getting big for football. So whenever I have my girlfriend over and we're watching t.v. he'll come out of his room shirtless and make sure she gets a good look. Then he'll sit down on the other couch smile at her and run his fingers up and down his happy trail. To which my girlfriend will snuggle up to me and use peripheral vision to stare at him. Then whenever he tutors her he makes sure he gets really close so that his arm rubs against hers and rest his weight on his arms to make them flex. Plus I caught them flirting while I was getting ready. He doesn't like her he just know she's important to me and loves to mess with me. Then one time she forgot her cell phone at my house and her friend sent a txt talking about Jason. So I checked the other txt messages and they were all to and from Lizzy and they were talking about Jason and how y he is. My dad loves Jason and is always talking about football with him and they're constanly throwing the football around the house or watching a football game. Plus my dad bought that kid brand new a pair of $109 shoes when he won't even buy me a new pair of skinny jeans or converse. My girlfriend and I used to make fun of all the jocks saying how stupid they are. However, now when I was making fun of Jason she got mad at me and told me he was smarter than me and if he's dumb than I'm down right retarded (apparently he makes all A's). She always stands up for him when I bad mouth him and it ticks me off. I hate my step brother he's ruining everything what should I do? My dad is to busy with him and he doesn't want anything to do with me. Even though I don't want much of anything to do with him either he stopped giving me allowance and now instead spends my allowance money on a gym membership and he and Jason are clearing out the attic so Jason can have a weight room. Even though it used to be mine up their.