Make sure both are neutered. They may just be playing really rough--some kitties do. Or fighting for dominance. Cats don't have "brotherly love". As adults, that's a human thing. If they really are fighting "for real" it may be time for either behavior modification, or finding one of them another home. But my first guess is that they play really rough, and then one or both get mad and accelerate the fight. Now THAT is something siblings do!