Think about where you can better set your own boundaries. My husband, a 5, is much better at boundaries than I am, while I can be a people pleaser. His boundaries seemed cold/difficult to me at first, but eventually taught me how to set my own boundaries and ultimately be happier, for me. Some ideas: set up filters to auto-delete anti vaccine emails, state your boundaries directly to her/husband as your own, you confidently + kindly state what you're able to do over Christmas, and what you're not (and stick to it, feel free NOT to go on the trip he's committed to). Be willing to be the "bad guy" with the boundaries, or else you're asking your husband to do something you're not yourself willing to do.