My W's grandfather and one of her brothers have made crude and significantly negative comments about me. In my life, I do not associate with people like that. Maybe I will forgive them. I have to some extent. They do not see the whole story or they are simply trying to help my W feel better. Still, the comments were out of line.
In the mean time, understand that if your boys go to the meal, your SIL could make comments that you will not be there to deflect for the sake of your kids. And as you say, your BIL is still good company.
Do what you will, but if that were me and the offer was there, I would go just simply to show my kids that I CAN "man up". Be the role model that you wish your kids to have.
