I know this is a difficult situation for you; family members can really cause problems if they are not being supportive. If you know that your MIL is in this category, you HAVE to bring your own food to eat every time you visit her. You CANNOT trust her to cook food that you can eat. Even if she did not use any gluten-containing ingredients in her chili, for example, she's still cooking in her gluten-contaminated kitchen with her gluten-contaminated pots and pans and her gluten-contaminated utensils.
You have to understand that it's not just a matter of preventing you from having a stomach-ache; the more you expose yourself to gluten, the more likely it is that you will develop additional health problems in the future, including other autoimmune diseases and/or cancer!! This is serious stuff!
Your health has to come before your MIL's feelings. If she doesn't understand, she doesn't understand. Here is a great website about
going gluten free: How to Start Living Gluten Free
. It's quite lengthy but VERY helpful. You might want to print out part 4 for your MIL; it's all about
eliminating every speck of gluten contamination from your kitchen. Also, see this article for why it's so important to stick to your diet: Zonulin and Celiac Disease
. Zonulin can literally "
open the door" for other autoimmune disease to develop.
Celiac disease is nothing to play around with. It can even attack your bones
and your brain
. The sooner you let your family know that you won't eat gluten under any circumstances the better.