I lived with this for 15 years (my brother is 15!). Believe me, it will get better. My brother has his problems, he has ADHD and Aspergers, but it still doesn't make it any easier. I know how it feels to always be the bad one! I'm at uni now, and not having the best time at the moment, but I am happier here than I would be at home. When I do go home for the weekend it's like being 10 years old again. It can be really hard.
When you get to this age, no one really empathises with the "My brother is a pain" thing anymore, but it can really get you down. It does me. I found that I had to just detatch from the situation. I started to absent myself from the family, I just didn't get involved anymore, I stopped interacting with them. I don't know if that was a good thing to do or not. I wouldn't necessarily advise it because it caused everyone a lot of pain, including me.
I think if you do just focus on Uni that will help you get through the next few months. Now, with a few hundred miles between us, my relationship with my brother is a lot better. We chat on MSN all the time. That really helps me, because I know now that I haven't lost him forever just because we weren't close as kids.
I don't really have much advice. I'm not much help! Just know that one day very soon you will be an independent adult, and this won't be an issue anymore.
Hope that helps a little. I do know what it feels like. You can PM me if you get so stressed you need to scream!!!!