mil upset with not inviting her whole family

There probably is nothing you can do. You have perfectly valid reasons (especially for the cousin who has abused/harassed you) but she chose not to listen and throw a hissy fit instead. You need to put your foot down and refuse to speak to her about this.

Or tell her she's free to throw her own separate reception for those people, but you and your FH will not be attending it. (But she will probably have a tantrum over this too.)

Suggest that she organize an annual family reunion potluck picnic at a local park. That way everyone can be invited and it doesn’t infringe on your wedding.

Just tell her too bad, so sad lol. It’s not her wedding🤷🏻‍♀️ If she wants to sulk she can just stay home

That's an insanely huge family! We ran into the same issue with my Greek and Italian sides of our families where both our dads want to invite all their cousins, and our guest list went up by like 50, and we originally only wanted about 100 people. Our parents are paying for the wedding so we felt forced to invite them.

Don't reason, explain, justify, or argue with her. You can't persuade someone with no decency or common sense. Keep the matter closed with her.

I feel like an explanation isn’t owed to anyone. I believe if someone wants to cry over such nonsense, then let them! Why is it some people don’t know how to act when it comes to weddings or funerals? I swear my saving grace is this site, because I literally do not speak to ANYONE about my wedding plans at ALL so I can avoid such things. I learned very early in the process that some people will do anything to rain on your parade and find the dumbest ways of doing so!
You just keep your wedding on track and do not discuss details with anyone that doesn’t need to do anything more than show up. Invite people who YOU want to spend the day with and don’t apologize to anyone for it.

I'm sorry if these ppl did these things and he feels strongly that he doesn't want there then that's it. Ppl has stop babying ppl bad behavior and dont want no problems nor embarrassment at your wedding. And your SIL should understand nor should she want that type of behavior. I have family and friends who get drunks and wants to fight or hit on every female and gets into a fight because she is with someone. Follow your feelings and your FH GOOD LUCK

"Cousin assaulted both your son and me. He's not invited. End of discussion."

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Also, in my opinion, this perfect message should be delivered by your FH -- his mom, his drama to shut down.... I've been happily married for 35+ years and H and I have great families that we both get along well with, but we've also always been the primary point of contact for our own side -- especially when there are sticky situations to navigate. You deal with your family, he deals with his, and set your boundaries now. Good luck!

Tbh FMIL should consider seeing a therapist if she's crying for days over someone else's invitation to her son's wedding, especially when that person has assaulted her son and his future wife. I agree with others that FH needs to be the one to speak to her/ his sister and say that the cousin isn't invited, end of discussion. It's your wedding, so you get to decide the guest list, and assaulting the bride and groom is 100% grounds for someone not being invited.