Just don't. Look, anytime someone doesn't want to do something, it's always going to create some drama whether you intended it or not.

Why are your coworkers in your personal life this much? They have no business knowing anything you don't want to tell them.

My initial thought is that it all seems very dramatic for a "petty" grudge, so I suspect you've only told part of the story. That said:

1. You sound caught between your mother and your brother, which isn't a fair place to be. Neither your mother nor your brother should have involved you in their personal issues. If they want the best for you, they should both want you to have a healthy relationship with the other person, especially if their disagreement is over something petty. In this particular example, your mother should be encouraging you to attend your brother's wedding. If she isn't, why not?

2. If your brother is the kind of person to hold petty grudges, your mother is unlikely to be the 1st person he has issues with. Is he known for holding grudges generally? If so, you are allowed to not want to be close to him.

If not, why this grudge? Who told you the details of the "petty grudge"? Have you heard both sides of the story? If you've only heard your mom's side, maybe you should talk to your brother.

3. If you've decided not to go to the wedding, then at least RSVP no. There is no reason to be discourteous. Not RSVP'ing could seriously damage your own relationship with your brother and/or his future wife, which you may one day regret. No need to slam a door closed unnecessarily.

4. Simply tell your colleagues you can't attend the wedding. End of sentence. You don't have to tell them why.

Man, if I had that kind of fucked-up enmeshment at work, I'd be looking for another job.

You don't owe anyone an explanation. You can just say that you're not coming and leave it at that. You can tell them that you have your reasons and you wish not to talk about it but don't lie. That just makes you the asshole.

Its impolite of your brother to not invite his mother and its impolite of you to not go. I think it would be best to say it so to him.

Just tell them what you wrote about your mother and your brother. If they don't want to understand, ignore them.

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.

Grow up.

Reply to the RSVP with a polite "unable to attend."

Tell your coworkers "I am unable to attend."

Anyone who asks more than that deserves a slap and a discussion with HR.

Fuck 'em. It's your life. Live it.

Go and RSVP +1 and bring your mom

Step one- the coworkers need to fuck off. Start with a polite, but firm “it’s none of your fucking business” and get less polite the more they push.

Step 2, if your brother is trying to weaponize your coworkers, tell him to fucking stop. If he’s just coming around, be somewhat more polite and still tell him to stop. If he switched to “I’m just visiting [the coworkers] your manager probably would like to know- ex employees still coming round is never a good idea even if they left amicably.

Step 2b (or maybe step 3) tell your brother that you don’t want to be drawn into whatever is going on with him and your mom, and that’s why you can’t go. If he’s continues trying to pull you in, you can tell him to fuck off too.

It sounds a lot like your brother has issues with boundaries. Set your boundaries and enforce them. Also don’t let your mom pull you into it on her side, either.

Too late to stop the drama by the looks of it.

Move to Peru.

Who gives a fuck?
If they want to go, they can go. Otherwise just say "family issues" and leave it at that. Fuck anyone feeling like you have to go to make them happy.

Explosive diarrhea.

One option I'm not seeing presented is to politely show up, offer congratulations, and then slip out early before any group photos or anyone can pull you aside to talk drama

Or if that's not doable just don't go. Any coworker harasses you for it, report to HR.

Talk to your Mother about this.

what is the grudge about? cant really put it in context without that info, just ignore, it give an excuse like you have work that cant be PTO,.

just lie, if your brother is petty, you can be too.

What's the "petty" grudge?

Why do your coworkers know your brother so well?

I guess it's rather how to create the less drama possible rather than doing it without drama.

You could lie about some other important thing that day (like a medical appointment that had to be rescheduled that day or a friend or family member living far away coming that day) or saying yes to everyone and playing sick on the mariage day.

You could say to everyone that you're not going if your mother is not going, but this is probably quite a drama generator. Another less direct version is to plan something woth your mother that day and use this as an excuse, but passive agressive strategies like this can result in drama too, though not always in front of you.

you wont go, you just want validation. get positive covid papers from the doc.

Vehicle issues 'last minute'

Fuckit, give yourself a flat tire, this ain't difficult..