My MIL told my fiancé it's "a slap in the face" that he didn't choose his brother to be a groomsmen. I responded to her that no role in someone's wedding is a slap in the face and she still seems pissed off at us. We chose for our siblings (his 2 brothers and my 2 brothers) to be ushers because they know our family making the task a breeze and it's equal for all siblings without his entire side of the bridal party being brothers. Is this some old tradition she's holding on to or just another thing for her to complain about? I don't want to hurt anyone's feelings, but I think my fiancé gets the final say on who he asks to stand next to him, not me or her or anyone else.
Re: Groomsmen
If you are close with your siblings, they should be in your wedding party.
That said, it's 100% your FI's decision who he invites to stand next to him. It should have nothing to do with his mother's opinion. It should also have nothing to do with who you decide to invite to stand next to you. If he wants his brothers, he should have his brothers. Excluding them because you don't want to include your brothers or even sides is just mean.
Last, you shouldn't have responded at all. This is his mother and he needs to do the talking. This is no place for you to butt in.