I feel for you as I am in a similar situation. My daughter may have celiac disease and I took her and her brother to see a chiropractor who practices applied kinesiology (muscle testing) My daughter was told to stay away from dairy, corn and anything with gluten. My middle son wheat, dairy and corn and the oldest wheat and corn syrup. It is difficult to get people to understand. I bring food for them wherever I go and if I send them to my mothers I give her strict instructions to follow. She is pretty good about following my orders. My in-laws are very much like yours but again as long as I bring what we need to eat they are relatively respectful. Inquisitive and don't seem to get it but pretty respectful.
I have a cousin who lives on a farm, has had her four children at home with no one but her husband to help in the birthing process. She does not let her children have any junk at all period and she does not even let her children go to the bathroom with anyone but herself or her husband. While I feel this is somewhat extreme, especially since her mother my aunt, is a beautiful person who understands the importance of health and nutrition but I also think that if you have people who are influential in the lives of your children they need to be on the same page with you. If they cannot respect your beliefs and what you feel is best for them then possibly they should not have time alone with your children. Supervised visits only and or you pack their food or bring a dish when you must have dinner at their house. I think as peanut butter points out if you bring a lovely dish of roasted sweetpotatoes and vegetables to share how can they pass that up. My husband loves a dish like that and he is definitely not vegetarian.