How families can stick to their eco-principles without starting a seasonal civil war. Sure, you could just grin and bear it for a few weeks or a few meals, but these are life-long relationships. And these occasions offer unique teachable moments—a chance to talk about concerns well worth sharing with those we love. You may have better luck with these conversations with your own mother than your mother-in-law. Be realistic. Some ideas:

When it comes to things that cross your line, try playing the humble novice.
If everything you suggest elicits the response, mention that you’ve heard that the damage things like X do doesn’t appear for years and then can be difficult to trace to its cause.

Give advance warning whenever possible so expectations are clear, whether it’s about gift-giving no-no’s, dietary requirements, or wearing shoes in the house if you’re bringing a crawling baby to visit.
Take breaks.
 
If you prefer to let things slide in the interests of family harmony, that’s ok. Brief annual exposure to many products and materials is often not a big deal. A few days of synthetic cleaners or eating candy with food dyes likely won’t do lasting harm. The younger your kids, however, and/or the more toxic the material, less tolerance is prudent. Ultimately it’s a judgment call parents have to make. Remember to have these conversations with a smile and affection. It will make for a much happier new year.