It's also occurred to me that it might be helpful to consider what your family holiday celebrations are like -- from your son's and his significant other's perspectives as best you can. I know we all struggle to spend time with DH's family for many reasons, which they are completely oblivious to despite some gentle and direct comments on our part over the years. They have rabid political views and attempt to force them on everyone, including us, at every opportunity. As our daughters have grown to voting age and approaching voting age they amped up the attempts to indoctrinate -- if so much as a simple question is raised to ask for clarification on a single point of the harangue they immediately want to drop the subject and say, "well, let's talk about something more pleasant." We're not talking a baiting or challenging question, we're talking, Grandma, you said X, what is that, I never heard of that before?
That's where we get into the real minefield. No discussion of television, movies or books will work. They refuse to attend a movie theater, only read books surrounding their political views and TV is the same, with the exception of HGTV. Perfect, let's discuss something wonderfully banal like DIY and home improvement projects! Not so fast there, Quickdraw. Only specific HGTV shows or types of home improvement projects or styles are worthwhile. Their discussion of anything out of their comfort zone is minimal and cutting. You literally can expect a tongue lashing for suggesting the wrong color looks pretty in a bath or bedroom or the wrong type of wood for kitchen cabinets.
Vacation discussions are either immediately torn apart as too expensive or ridiculous -- Chicago, Disney, Las Vegas, etc. If a destination somehow manages to pass their approval it becomes a one way lecture on their expertise touring there -- frequently concentrating on a cooler, bologna sandwiches twice a day and the best routes to get around. They may have asked, how was your trip, what did you do, but answer at your own peril -- and not on their time, because they have stuff they want to tell you about where you went even if they've never been there.
Sports are nonsense. Family dinners consist of laundry lists of ailments and why they can't eat a list of ingredients that fluctuates within a single meal (when I'm cooking), or seriously overcooked meats and skimpy to the point of nearly non existent sides (when MIL cooks). Table conversations start with an inquisition of my daughters about their studies as lead-in to yet another round of tales of BIL's natural brilliance in spite of teachers who tried to thwart his superstardom. (BIL is in his mid fifties, supported by mom and dad and has yet to start a career or even hold so much as a minimum wage job despite a college degree -- chosen for having zero possibility of gainful employment in the field by his own admission.)
You may think it sounds extreme, but sadly I can tell you it's absolutely true. It is beyond challenging to welcome the prospect of spending time with people who are unpleasant when you discuss literally any topic under the sun -- all while complaining you never spend time with them. OP, I urge you to take an outsider's look at what your own family celebrations look like and try to evaluate as objectively as possible what kind of an experience they really are for your son and his SO. They may not be as extreme as what I've outlined here but these types of things are common in extended family dynamics. A lot of singletons dread the holidays because they get quizzed about their love lives and family holidays often focus on doting on the small kids, possibly ignoring the lives of childless family members as irrelevant. These types of things are worth considering if you really want a solid ongoing relationship.
