I will fully admit that there is a trend in pop culture to cast vegans as some sort of holier-than-though crusader type. I will say in this thread at least, people have been good about using counter examples that indicate the trope is just that, a trope. Not to say there aren’t people that fully live up to it!
I tend to think, even as a meat-eater and lover of tasty critters, that one day it may actually evolve to the level of our current feeling about public smoking though. When I was young (and I’m younger than a lot of posters), I remember my grandparents and their generation smoking through the holiday around everyone else, and almost no one would object.
By the time I was a teen, it was expected that the family members who smoked would go around the corner as a group and light up, so no ashtrays or smoking in or around food.
By the time I was on my own (and my grandparents generation had largely passed) there was a feeling of pressure on the smokers to ‘do it at home, not here’ during family events, and to be out of sight and smell of the kids if you had to (go outside, or even down the street, not just a corner of the house).
I’m sure to a degree that those smokers of yesteryear felt that the anti-smokers of the past were being judgmental pricks, but attitudes towards acceptable risk and behavior change over time.
Will various flavors of vegetarianism and vegans become so predominant in the future that the majority just shakes their head at the fools of the past in wonder? I doubt it, but it’s possible. And if so, likely for sustainability / environmental issues rather than a philosophical sea change. But it could.
Still, as put clearly by multiple posters, most recently by @LSLGuy, a part of any large (especially family) gatherings, is the unspoken or spoken understanding that this is an event to celebrate the family and come together. Very few families live, work, and believe in perfect lockstep - so to facilitate the joy of all, everyone agrees to minimize conflict. If you cannot (as apparently the OP’s family member cannot) then it is best to not attend.