Let it be. No judgement. That's the first place to start. The more resistance you create by trying to "teach" someone, the more it persists. Just bring a great vegan dish along, eat nothing else with no apology and no need to say anything. IMHO.

Here in Thailand if you turn up ANYwhere wearing only simple pure white clothing, you will only be served vegetarian-vegan food, the premise being that you are making merit. Never questioned by anyone and the gesture applauded. I think most of it is cultural, rather than your personal family.

Enjoy the moments. Focus on what's great and you will get more of that.

Happy & Healthy New Year to you!