I recently had the reverse problem with a vegan dinner. We invited some new neighbors over for dinner & drinks. They told me they only eat "plant based". I interpreted that as meaning vegan, and felt I could accommodate them. It was all food that I was happy to eat, except I would usually include an animal based protein.
For an hors d'oevres while we were having drinks, bruschetta on toasted crouton, fresh tomato, onion, olives, basil, oil, etc, with a balsamic drizzle.
For dinner, a big (and fancy) salad, with homemade vinaigrette. Homemade semolina pasta (no eggs), with fresh truffle and mushrooms and olive oil - yes, fresh truffle.
For dessert, a poached bosc pear drizzled with a chocolate balsamic vinegar.
I thought I put together a fabulous meal for company with dietary constraints. Fresh truffle! Well, until they arrived, they didn't say that they only drink water. I noticed that one of them wasn't eating any appetizers. When we sat to dinner, she barely picked through the pasta dish. She did seem to enjoy the pear, and by the way, ate the entire bowl of grapes that were sitting on my kitchen counter.
It turns out, she is not only vegan, she eats no grain or oils. Apparently, to her, plant based meant only fruit and vegetables! And of course there was no mention of this when I invited them. But flour to make bread and pasta is a plant! She eats no 'starch'.
We saw them again a couple weeks later, and she was sure to let me know that the olive oil in my pasta and salad gave her diarrhea for 2 days. I apologized.................
Your SIL needs to learn some manners......