On threads like this, I think most people try and understand why it might be the case from the other person's perspective.
We don't actually know what any of these large family gatherings where everyone has a whale of a time involve.
For example.
Are their family get togethers quite sedate? Food, small amount of alcohol (no one has ever really come close to being drunk), games, chat, laughter with everyone driving home at the end of the night. A couple of people like to sit around discussing obscure Russian literature, depending on who attends, there is sometimes are air of intellectual competition but its ok because it's all good natured and and polite and just people exercising their brains for half an hour or so. Everyone has a whale of a time and it would be a shame to miss out on that.
Or do they look like huge amounts of food, huge amounts of alcohol where all the adults get very drunk, slightly reckless behaviour, unsupervised children running riot and hanging out of the upstairs windows. Hosts who insist everyone stays even though the house isn't really big enough so everyone just muddles through and chaos ensues. Some people love all that - big loud, happy, rambunctious stuff. Stuff gets broken, accidents happen and its all just part of the chaotic fun of a party. Everyone has a whale of a time and it would be a shame to miss out on that.
Or does it involve music, games, chat, conversation, an ill considered karaoke session, maybe a christmas film. It's all adults so people get a bit tipsy but no more than that. Everyone has a whale of a time.
Or maybe the hosts ae functioning alcohhoics who consider everyone to be having a whale of a time if everyone is drunk, loud and crashing about and an argument is par for the course. Sometimes someone has eventually dramatically left in tears and other have gone to find them... but, when they dust has settled, they ever that until x,y opr z happened, everyone was having a whale of a time.
Those all describe get togethers hosted by me, my sibling, my parents, my exh husband's parents and my current partners various extended family members on all sides and most of which I've attended at various points.
And I can tell you, family or not; cousins or not, the quiet non drinkers don't want to be around the loud, aggressive alcoholics; the ones who parent their childen well don't want their children around the unsupervised ones because they're not having good cousinly fun, they're running riot and getting hurt and the ones who like to get on the garden bucking bronco pissed think the ones who engage in intellectual debate are boring.
The bigger the family is, the less likely it is that everyone enjoys similar things and the less likely that family gatherings are to everyone's tastes.
We don't know what the OP's family gatherings look like and everyone who hosted the above parties did so because it was their idea of a fun night.
In the end, no big family get together ever happen/ed and people just get together in smaller groups with the family members they most aligned with.
The brother and sister in law could just as easily be the intellectual thinkers who want to discuss wine and holidays to Lake Garda as they could be the ones wondering why no one else is getting pissed and they're all so boring.