If your MIL is a vegetarian then yes, she should expect a vegetarian meal. You don't serve meat to a vegetarian. Ever. Anyone with an IQ above 20, no - 5 even - knows this.
Would you invite a Muslim or Jewish person to lunch and serve them pork?
Would you invite someone with severe nut allergies to lunch and serve them a meal with nuts in it?
Would you invite a person with Celiac disease and serve them a lunch full of gluten?
Would you invite a diabetic to lunch and serve them a lunch of only sugar?
If you genuinely can't see that you are selfish, vile and wrong inviting a guest to lunch but serving them a meal you know they cannot eat, and expecting people to agree with your selfish, vile and batshit behaviour, you have another think coming. You should be ashamed of yourself.
If this was posted in AIBU it would have a 100% YABU vote.