Caregiver - Needing some advice

One of my sisters has suffered with migraines since the age of 7. She lives with my elderly parents, 82 and 88 years old. I moved in with them a year and a half ago mainly to help my aging parents with ADLs, cooking, cleaning, shopping, Dr. appointments, ect. Anyway, I have helped my sister out as much as I can. I have had only a few migraines and know how bad they feel. I found out I had celiac disease, cut out all gluten, and haven't had any since. I was lucky that's where the headaches came from. I tried helping my sister more with the food she eats. She has to be on a low-tyramine diet, and in addition to that, she is allergic to dairy, pork, corn, and potatoes. She didn't know what a low-tyramine diet was. I just happen to see her notes from her doctor appointment that she left on a table and asked her about it. I researched on the internet and showed her. I don't understand her sometimes though because she still eats foods that trigger her migraines. Sometimes she will knowingly eat something she is allergic to, take one of her headache pills, then call in sick the next day at work. Sometimes she misses a whole week and then I see food in her room she is not supposed to have. It is frustrating when she does these things. I help with some of her meals but I have to cook gluten free (me), low-tyramine (sister), diabetic (dad), high blood pressure (mom) meals. I have showed my sister so many things but she will make no effort, even on the days she is feeling okay. Some of the meals are really simple, less than 5 minutes.

How do you help someone with migraines when they do not follow the instructions their doctor has given them.