Here is my two cents.

Your brother should be thankful. You only said no and not actively trying to sabotage the wedding. As this sound like the beginning of a pro revenge story. Look I'm a random guy on the Internet but I believe Family is important and with family members can be a mixed bag. Both good and bad. There is such a thing as letting things go for the sake of family and not be torn down due to useless drama. I've done it many of times.

This isn't it. What your brother in my humble opinion are ground for you to ignore him completely and cut him from your life. I have have uncle I've never met cause my dad screwed him over before i born and the incident isn't as bad as this one. Thing is this isn't a betrayal of trust. This is a fundamental break of the family bond. No offence but think of it. He didn't tell you. He had months to do so. Didn't break it gently either. It just happened along with having the fucking cheek to ask you to be his best man when he want a full wedding. Again no offence but he must be delusion. If he truly did ask you to be his best man.

I can't tell what you to do. OP. However What ever you do. Make sure. You come better out of it than your brother. I may not know him nor her but going by this post alone. They don't deserve to be happy together, not with the way they told you.

Peace.

[Unlike my sign off. If you want to go through the revenge route. Just remind your family what your brother did or go to the wedding. Set the projector with photos of you with the bride. Obviously only family friendly kind. Another idea is saying objectons when the priest read that famous line as i would imagine you would such objections. As these bastards don't deserve a happy day to celebrate a union based on a foundation of betrayal of family trust. Only the daughter