Gutmutter - My mother in law is the same way. Since my weight loss surgery, I haven't been able to eat sugar, fried foods, or foods high in fat. Literally everything that woman makes is full of sugar or fried. Even veggies will be glazed or something that makes it so I can't eat them. She's known for the last 30 + years how much I hate onions but she puts onions in everything possible. Before my surgery, I worked around that, but since, I haven't been able to eat anything she cooks. I truly believe she did it on purpose just so she could complain about how "picky" I am.
We don't have a relationship with her anymore, but I wanted you to know that you aren't alone in this.