I pretty much agree with MrTango. People have the right to get married when and where and how they want to. And their families have the right to be upset that they weren't included/informed ahead of time.
People who elope (or do any other big life event) without informing their families aren't rude. But they might also not be considering how their actions affect the rest of their family. I have several brothers and I can assure you that all of them have made similar moves in their past. And they been faced with an upset mother, and a father who is angry because Mom is upset. And they learned a lesson.
I'd be upset too. One of my brothers was so into surprising the family with a surprise announcement of his engagement that he overthought things and managed to miss the three days that every one was home for Christmas, and he "surprised" only half the family. I didn't find out for a month about the engagement. (Although I suspected something was up, based on a few things my now-SIL said.)
He was so caught up in "Let's surprise the family!" that he overlooked that he missed telling half of us.
I think some people get caught up in the fun! exciting! atmosphere of the Surprise! that they overlook the real goal, which is to let family and friends know of a big, important event in your life.
In the OP's shoes, I'd be upset. But I would also want as good a relationship
with my new SIL as possible. So I'd rant a bit in private. And then I would put on my company face and greet the new family members at the door with a smile. I'd have a bottle of champagne on ice for a toast. I'd try to find out the age of the child and maybe have a small "welcome to the family" gift for him--it's not his fault that the grown-ups he's related to are acting this way. I'd send a card. And I would have a small gift that I'd probably give to them as they leave, so as not to make too big a deal of things at the Easter dinner.
Because just as the OP's brother's choices have affected the OP's family, so will her choices affect her relationship
with her brother and his new wife. Unless there is a toxic background, I'd err on the side of "doing the right thing" and trying to create a good relationship