Is there a cultural aspect that is not being stated? Because if there is a cultural expectation that you attend the wedding, that’s important context.
I also get the sense you are on the younger side. Your brother can’t “turn you against your mother” without your consent.
If you want to go, go. Talk to your mom and tell her it’s nothing against her. If she has a problem, that’s her issue.
If you don’t want to go, tell your brother you don’t want to go. If he gives you shit about it, again that’s his issue.
I don’t understand why you care about your co-workers’ opinion. Just tell them you don’t want to go. If they ask, tell them it’s because your brother has a grudge against your mom and then let your brother deal with it. If they ask what the grudge is, tell them to ask him.
At the end of the day, it’s not your problem.
Unless it is because you’re leaving things out in your post.
But will those concerns turn into anything actionable?