Exactly this. ^ The chances of a big extended family being super-happy, and the Christmas being all shiny and lovely and festive and happy, are far less likely than one half of the family hating the other, and there being toxic vibes, toxic people, and bitchiness and discord!!!
If you've for a chance...and you can find it anywhere, watch 'The Family Stone' with Sarah Jessica Parker... (It's streaming on Hulu, or its' a couple of quid on ebay for the DVD.) Not exactly the same as the OP's situation, but along the same lines...... A man wants his girlfriend (who he hasn't been with for very long,) to come and spend Christmastime with his huge family. IIRC, it's 2 sisters, 3 brothers (1 brother has his partner there, and 1 sister has her partner and child there,) then there's mum and dad. About a dozen altogether.
The family are aloof with SJP, and a bit cold, and the youngest sister is vile to her. They thought THEY were a lovely happy jolly close family, but they were actually toxic, narcissistic snobs who looked down their nose at everyone who wasn't in their 'family circle.' SJP says the wrong thing a couple of times, and they lay into her, bully her, and treat her like shit, and favour her more gentle, compliant, placid younger sister who comes to stay after a couple of days. (She asks her to come to give her an ally/have someone on her side!)
The OP @Ifeelabitweird and her family remind me sooooo much of this film.