Their is no 'fairness' if the dynamic between your future dil's parents dictates what you are allowed to do, and what hasn't been mentioned so far is that if you roll over on this, you will set a precedent for the future so that means all other normal family gatherings (celebrations of children's birthdays, school achievements, eventual graduations etc., etc.,) will follow a pattern that has been long established. Honestly, I wouldn't keep quiet, I would point out the long term folly of everyone enabling the selfish behaviour of grown adults, and highlight that the future hinges on the example they set now. Have they thought that this could lead to their children never being able to have their different sets of grandparents in the same room as them I wonder - it's utterly ridiculous. And yes, as others have said, your son is now an adult but just because we're getting older doesn't mean we should stop modelling rational behaviour. You could suggest to your son's partner that she ask both parents to visit a couples counsellor precisely so that somebody neutral and skilled in teaching couples whether together or otherwise how to be less self-centred and co-exist in situations where other people could be harmed by their behaviour.