Advice request: How to do with stupid relatives?

[Alex]

Tell the brothers you're sorry but they didn't give enough notice and while it sucks they're asking you to be rude to others because of it.

Then call your MIL and apologize for not being able to attend because of the conflict and assure her you'll talk/see her soon and happy birthday.

If they freak when they don't get their way, just like children it is probably because by freaking they usually get their way.