Hi Deb,
Boy have I been through such similiar situations!! I've been out with MIL too many times to count when she would ask if the kids could have "butter" on their bread and when we politely remind her "no" she rolls her eyes and sighs heavily. We homeschool also so she already thinks we are completely nuts. But over the years she's actually backed off and doesn't do it AS MUCH. Although, she still has her moments.
One time after much frustration with her, I typed up a list of exactly what the kids could and could not have. I portrayed it as though I was trying to help her out. I've also given her resources (which I'm sure she's never read) but it at least lets her know that we're not doing this just because but for REAL reasons.
It's really hard to find anything to eat at Chili's. I believe the black bean patty has dairy in it. Just know that you're not alone! It can be very frustrating dealing with family members.
Kelli