And before you downvote me to hell, read my thinking please. I 100% agree with "your wedding your rules".
But it your brother's kids, and they are young kids, so their parents' input matters. He obviously can't force you to use them all, but he can politely decline to have you use any if he wants. Use of his kids, or any honor in a wedding, is not a requirement, but a request. And that request can be politely declined.
And if you are nervous it will descend into a "situation" then you shouldn't use any of the kids as using some only creates a rift that can be exploited for the situation, and future relationship challenges with you in the middle. For your own sake, I'd stay away from all of them.