Med school, I know little about, but sibling rivalry in schools is another story!
As you are the more studious sibling, I think this will be harder on your brother. OTOH if you do get into the same places, in the end I am sure it will improve your relationship. I have a sibling very close in age who is interested in the same things I am, but find that we really have very little competition - her favorite class might be chem, mine might be calculus, and if we're taking it together it's a chance to be helped by someone who really doesn't want you to fail!
It's a great opportunity and I'm sure your parents are really proud of both of you. It also ensures you always have an ally. Just try not to lord it over him if you do better, to each their own.
Remember that once you get to the clinical part, you won't be together anyway.