Honestly, and I already know I’m massively against the consensus here, but you should be the bigger man.
Tell your brother that you disagree with him but that he’s family, you’ll always love him and that you’re honoured to be a guest at his wedding.
If you want to draw a line and say “let’s not talk about politics though” or something like that, then absolutely do.
Don’t have all your family and his friends talk about how you’re refusing to attend because you had a fall out over politics, you’re not going to appear principled, you’re going to appear petty. You’ve made your point to your brother, now move on.
You’ve applied the label of fascist and Nazi yourself, nowhere in OPs post does he define his brother as a fascist or Nazi. Left Vs Right political differences doesn’t mean you automatically get to apply labels like commie or Nazi, a lot of people on Lemmy need to grow up.
OP bought up the Musk debate with his brother knowing that his brother is a right winger, then OP got upset when his brother acted just as you’d expect. He poked a dog with a stick and then got upset when it barked.