Having read all of your comments, I personally think your mil has really damaged your relationship with her other sons and not going would be playing straight into her hands.
You saw fit for them to be godparents. So time to draw a line under the fact that these two, presumably at the time, young men, didn’t acknowledge your wedding formally. It’s done and tbh really is the same as you not officially sending formal invites to them. Idk why they didn’t attend the male celebration. Perhaps they were also hurt.
Or.... perhaps your mil was also pulling their strings as well and telling them not to attend because you couldn’t even be bothered to send them formal invites so you didn’t really want them to be part of the wedding party.
Therefore, personally I’d circumvent your mil. Put the past behind you. Send a lovely card saying you all can’t wait to celebrate their special day with them, plaster a smile, gush congratulations and let your kids have a fun day. God only knows, we could do with some joy after the past 12 months.
And if you really want to be the adult in the room, you could go up to the other bil and wife and congratulate them. Tell them, you’re sorry there was a misunderstanding between you all, which meant you missed out on seeing them get married and you hope everyone can put it behind them.
And when your mil comments on your accent or anything else, grey rock. She only makes herself look stupid.