he knows I have told him,he just thinks we all want to make them make up and that we would set them up and lock them in a room together or something
When people say they're not going to take sides, and they try and get the two "sides" to make friends, but they do actually think there are sides and one side was definitely in the wrong... That is actually picking a side. The side of the person who has done something wrong.
It sounds from what you've said like although you all thought your sister's comments were well out of line, and that your brother was justified in feeling how he does about what she said, none of you really behaved in a way that backed up what you supposedly thought.
Your brother avoided the rest of you because of the one sister, because he knew or felt that the rest of you wouldn't respect his wishes to not have anything to do with her any more.
And by letting him do that, rather than making accommodations for him in the beginning, of course it's going to look to him like you all took your sister's side.
Actions speak louder than words.
You can repeat until eternity that you thought your sis was in the wrong, you can keep inviting him to events knowing full well he won't turn up. But your brother is not going to feel like a part of the family again without you all making a proper effort to acknowledge his feelings, and the things that caused him to put distance between him and the rest of you.
Namely, the whole "we're happy for DB (the wronged party) to estrange himself, but we won't exclude DSis (the party in the wrong) on occasion so DB can still be comfortable seeing us" thing.
I can understand why your brother has pulled away from the rest of you, and he really has done nothing wrong by having a wedding without family. I'd say the rest of you have been wrong for not showing him proper support when he and your sis fell out.