You have to quit comparing yourself with anyone else; your brother included. You aren't your brother! I'd be smug if I were called "the human calculator's sister" -- they don't know him as a person either and being a "human calculator" is not something I would ever want to be described as? I imagine it is people who interact with him more anyway and that is always going to be the case; his friends and acquaintances are not going to be yours.

Find interests of your own instead of just those that are "his". Do you really like karate? Did you try ballet or tai chi?

Even school grades; they do not matter, they are something outside us trying to measure us against "average" and when you get to working in the adult world, no one will know or care what grades you made or did not make. I flunked international economics in college, do you think less of me? No? Why not? Your brother would probably have aced the course. Do I care? No, because it is not something I need in my life. Yes, I went to summer school to make up the credits I needed but that too was part of my life and part of my tasks and I learned, grew, and (now :-) enjoyed living that part of my life too.

All sorts of things will come into your life and all your experiences will be totally different from your brother's. You are not growing up with your brother, you are growing up alongside him; your lives are totally different. He cannot be better than you at being you because you are the only you there is.