If you don’t feel comfortable telling your family you could lie to your family and say that there’s something on your fiancé’s side of the family that’s you already said you would attend on the same date so you could get out of going to the wedding. Your family with still complain to you though. You could tell the bride that you don’t need to be the maid of honor and she should use that spot on her friends but you are very flattered that she is trying to include you in the wedding (if they still want you in the wedding party you could make up an excuse that you don’t like the attention of people looking at you). You could also write a letter to your brother and tell him how you feel about your history or ask to meet up to talk about it. He might even apologize

I’m sorry your put in this situation and I can’t imagine having to keep holding in this secret for so long