Having had numerous babies (ha!) and had my parents generally come in a few days or a week before the birth, then stay a week or two after the birth to help out and watch the other kids, I still can't see a wedding, even an out-of-state wedding, right around the birth, being something they couldn't handle. I'd need a backup plan for childcare, certainly (although that isn't really applicable to this story, as I think the OP said it was C's first). But usually right after the baby is born, you're in the hospital for a couple of days, and exhausted, and getting used to a new baby... it's not the end of the world if the grandparents miss the first few days after the birth. And that's being pessimistic, assuming that the baby was born as the grandparents were about to leave for the wedding, so that they had to miss the entire few days it took to travel to the wedding, enjoy the wedding and reception, and then travel to where I was with the new baby. Think of how much fun they'd have sharing the news at the wedding reception! The sister could send some pictures and hopefully somebody with a smartphone could show them. I don't think that showing off some pictures of the new baby at the reception to other relatives would totally hijack the wedding, anyway (because how much of the time at the reception are people actually talking about the happy couple? Probably not all that much, as most people enjoy the chance to chat and catch up, etc.). And, of course, if the baby didn't arrive until the day of the wedding, or something like that, the parents could enjoy the wedding and reception, then fly to the sister's city to see the new baby afterwards. I don't think missing the first two days of a new baby's life is *that* dreadful.
Now, of course, that assumes that the sister is okay with missing the wedding if she needs to. And I think that's the issue that a lot of people are concerned about. Personally, for a sibling's wedding (even though I don't really like my brother all that much), I'd probably do pretty much what it took, short of deliberately putting myself in a position to have birth in a different city without my doctor, to go to the wedding. I wouldn't love doing it, but I'd try. However, I think I'd tell him that, out of necessity, my RSVP would *have* to either be "maybe, if I can," or "no." If he couldn't accept a maybe because he needed a firm headcount, I'd need to RSVP no. But if he could handle a maybe (and my husband's as well), then I'd make it a maybe and try to be there if I could, assuming that I wasn't pregnant and within two weeks of birth, or had given birth within the week. Unless it was local, then I'd go as long as I wasn't in the hospital. (And actually, now that I've had 4 kids who were all late and had to be induced, I'd probably go almost up to the due date, assuming I didn't have to fly.)