My fiance wants his brother to be one of his groomsmen, but I don't want him to be one.
Would you rather have your fiance's brother at the wedding?
He's a good guy, but I don't want him at the wedding because he might make me feel awkward.
That is your family how their event should be. It's not your decision to make.
I really don't want him at the wedding since I don't want to spend an uncomfortable weekend with him.
This is your fiance's brother which means that they must be very close. Would you like it if your fiance told you that you couldn't invite someone that you've known your entire life? Don't force him to have his important event the way that you want it, because this is shared between both of you.