I have CF, and I would say your brother has a attitude problem. =-) How old is your brother? If he is in his teens, then i'm sure he'll just grow out of it, but if he's older than that, I think all he needs is a good slap in the face. =-)
It's true I have feelings that a lot of people in my life just don't understand what it is like to have cf, but I also realize I'm not the only one who has to deal with it. To be honest, I don't really know your situation, so I can't do anything more than just guess, but it sounds like your brother is just feeling sorry for himself. Not that there isn't anything wrong with that every once in a while, but when people are feeling sorry for themselves all the time, they forget about the people around them. I know, it must be very hard for you to see your brother with cf and not be able to do anything about it. Harder still for him to just brush you off like that. I'm not sure there's anything you can do to change things, but hopefully he'll come to realize how important you are. In the meantime, it sounds like you're doing the best you can, and I'd say even though he doesn't acknowledge it, it does make a very huge difference.