I think diets are almost like politics and religion, sure to cause arguments if they are brought up. If you keep quiet about what they eat, you can demand that they show you the same courtesy. I had some relatives that liked/like to bait me. Once they saw that I wasn't interested in debating, they stopped. I hope your relatives get over it and leave you alone. If not, you could always threaten to get factory farming photos to share with them while they are eating