I was also in this situation and we had concerns that some of my husband’s relatives wouldn’t attend. We talked a lot about what our response would be if his immediate family responded in the way you’re suggesting. If the OP makes the decision not to attend, she needs to go in eyes wide open about what that will mean for her future.
Seeing grandkids? Probably not, if you refuse to acknowledge the wedding of their parents. Would we spend the time and money traveling to see them on holidays? Unlikely, since both sides of my family would really like to see us and vacation time and money is a finite resource. Elderly parents ever coming to live with us? Nope. We would support them financially as much as we could, but they wouldn’t be welcome to live with us as family. Attending his brothers’ weddings? No way. Could you think of anything more humiliating?
Our take was that we’ll love them, and we’ll always have obligations towards them. But if they’re going to make a big show about not approving of or acknowledging our family, then they simply don’t have to be a part of it.

But we weren’t going to let anyone disparage our marriage and then go on as though nothing happened.
Fortunately, after the initial upset, it was a no-brainer for everyone to attend the wedding, especially after they talked to their priest. The only person who seriously thought about not going was my husband’s grandma, and my MIL talked her into it.