I have a friend who is currently in a very similar situation. She is intending to do something that her maternal grandmother would strongly disapprove of, and she has been instructed by the rest of her mother's family - who can charitably be described as "toxic" - not to tell said grandmother, because she is in poor health and the news "would kill her." Like you, my friend had conflicting feelings about whether to stay silent vs. telling the truth.

From what information you've provided, I suspect that your wife's family is pretty toxic too, otherwise they wouldn't be putting you and your wife in such an uncomfortable spot by trying to swear you to silence on your daughter's marriage. I'm going to give you the same advice that someone wiser than me gave to my friend: this is not your problem. Your wife's relatives are the ones who want to keep the news from Grandma, so let them fret over it. Neither you nor your wife have an obligation to tell Grandma the truth: just sit back and let your in-laws stew in their own toxic juices.

As to what you should do if Grandma contacts you directly, unlikely as that is, you still shouldn't tell her about the wedding (unless she's contacting you because she knows already and wants to yell at you, in which case there's no point). Doing so may get a momentarily gratifying reaction out of the bigoted old matriarch, but it won't be worth the nastiness that will ensue, and the pressure on you, your wife and daughter.