You've got a rough deal there. It sucks when parents favour one child; sucks worse when they make it obvious.
The only thing I can really suggest is to try your best to be wonderful without their constant pandering. You seem pretty mature and independent; it doesn't look like their coddling of your brother is doing him any favours. When it's time for you to go to college, get a job or just move out then you're going to be the one who can handle it while he can't cope and needs your mother to visit every weekend and do his laundry. You could try to discuss it with your parents, but I'm not sure that would change much and it would be very upsetting for you.
I knew a girl in a similar situation to yours, once (the mother is painfully clear that she favours her son over her daughter) and she reacted by going completely off the rails, presumably because she'd rather have negative attention than no attention. She's sixteen now and has been smoking, doing drugs, drinking and sleeping around for about three years. The fact that you're not like her shows that you've done the best you can with a bad situation, in my opinion, and in the end you'll be the one laughing when you've build an adult life of your own and your brother's either struggling or just still living at home.