What the fuck is wrong with people where allergies/intolerances/etc are concerned? My BiL was giving me shit just last week because I wouldn't eat a piece of chocolate, and expressing skepticism when I said it would make my headache 10x worse.

I think there's an important addendum, here. Under no circumstances should your daughter be left alone with her grandparents, or anyone else who is not actively protesting MiL's behavior. It's not safe.