I think buying for one child is understandable if you do not know the second child but the original post states that the party is for brothers so it's highly likely that the OP knows both birthday boys.
Not always - I know DS's friends have siblings, but he doesn't "know" them. Plus, DS isn't usually friends with the sibling. I can think of one family where he's friends with both children, and even then calling the younger sibling a friend is a bit of a push, although they will sometimes play together.