What have my dad and brother done exactly apart from not talking to her. They were going about talking to others
Well they, and your sil, couldn't be amicable or even civil for the day. They didn't have to chat.
People do this all the time. But long time feuds to bed for a short period of time. For the sake of an event for someone they love. I have done it.
I think this, for you, isn't just about the wedding. It's about you still hurting over what she did. Even though she didn't do it to you. Its still hurt you.
What would you have liked the alternative o be. Her sat there all day/evening becoming increasingly uncomfortable, with people noticing? For her to ask your sil to stop staring at her? How would that have ended? For her to come to you and say 'this is too uncomfortable and sons partner keeps staring and I am worried it will end up in a scene'.
Or, bow out, leaving everyone to relax without making it your issue to deal with?
There's a reason you don't believe her reasons.
It is coming across, wether you mean or not, that you think she should have sat their uncomfortable as a consequence of what she did.
I really dislike cheating. But I also think that when someone makes a mistake like cheat, they should have to pay for it forever. A bad act doesn't make someone entirely a bad person.
If your mum is generally a bit of a shit person, aside from the cheating, then maybe you need to stop trying to force a relationship that's never going to happen. You may have wanted your mum there but I she isn't a decent mum or person, then maybe you need to take a step back and keep her at arms length.
Or she is generally a good person, in which case you should really be more understanding of her position at the wedding and continue the relationship. You can dislike something someone did, but still love them and understand what they did isn't their usual behaviour.