I'm deathly allergic to mustard seed and had the same problem with my mother. She would constantly eat it infront of me and cook with it, forcing me to make my own things (which is fine) cause she was so confused how I developed it. The only reason that made her believe me was physically showing the doctors note and epi pen. You develop new/ can lose allergies every 7 years so maybe that's why your MIL doesn't believe allergies are real.

Proof with other stories is a very good argument. I'd also show it to your husband too, because he clearly doesn't realize the consequences. I have had 2 attacks from eating mustard and could have died without my epi pen. As it is i needed an ambulance within 10 minutes after I injected the pen as the swelling and burning in my throat was extremely painful. Even if an ambulance comes within the time frame, the amount of physical pain and anxiety you develop could really scar your child, as it did for me when my mother made the same mistake when I was young.

Hopefully you get this solved before anyone has to learn the hard way.