Your family. Your house. Your rules.
I it explain to people like this (we deal with life threatening allergies as well but it helps get the main point across).
"Mom and Dad (inlaws- let your spouse explain it), I love you and I know you love us/the kids. I would not let you leave a gun sitting on our counter. I will not tolerate the same thing but with food. You will be here a week. You can eat out if you so choose but in this house, we eat this way and that will not chance for the health and safety of our entire family."
The end. No need for more.