My understanding is that you've got an extended family of 40+ people. MIL used to host, but can't manage it, and now a couple of sets of siblings have sort of taken over the organizing, at MIL's place, but it's poorly managed and they complain a lot. And, because of the size of the family and inlaw commitments, the same set of MIL+siblings host *three* events the week of the 25th, pressure people to show up to them all, but don't set dates and times until about a week before, with great whining about the stress. Several of the other siblings, including you, are fed up, but you know from previous history that even tactful attempts to suggest a change will cause drama that is remembered and resented for years.
Is that a fair summary?
I think PPs are right in that there isn't a magical set of words that will change things without drama.
What I would suggest is a two stage approach. The first is to suggest a change as tactfully as possible - you appreciate the work they're doing, but you know it's getting harder and harder as the family gets bigger, so what about rotating the hosting from year to year? You'd be happy to do so next year.
This probably won't work, from what you've said, so the the next step would be to change your own behaviour. Next year, decide how much you want to participate in, and act accordingly. Go to one of the events, on a night that you're free, but don't worry about keeping the whole week clear until they make up their minds what they're doing. Skip the rest - you can say that you've found that too many events makes you/the kids cranky and stressed, so you're pulling back (or that you've already accepted more timely invitations). Ignore the complaining of the host-martyrs, make you dish to take, and leave a bit early if it's too chaotic.