My advice for your situation is to recruit the rest of your family to help you educate her. If it is just you telling her she may discount you as being overdramatic or something. If your entire family is united in saying things to her (like "Have you washed your hands? We have to be very careful that Principessa doesn't have an allergic reaction. We don't want to ruin a family gathering with another trip to the ER" ect.) eventually she might get a clue.

If you know she is coming to something maybe your Mom would ask her to bring something fairly safe that she can't mess up too badly (depending on what you are allergic to maybe salad and bread or something like that). Or if she is extremely dense your Mom could have her put her food directly on the table with designated utensils so that it can't cross contaminate the food in the kitchen. Then you make your plate of food from the kitchen before things ever get served and you will know it is safe.

If you have had a good relationship with this relative up until now you might try either having a conversation or writing a letter to her about it. If you try this route I would be sure to try to assume that she has the best intentions in the world and give her the benefit of the doubt. People who don't have food allergies have a lot of trouble getting their brains around it sometimes. They don't see food, any food, as a potentially lethal weapon even though to us it may be. You see her walking in with poison aimed specifically at you and then wondering why you are upset. She probably sees it as a favorite recipe, that oh yeah, you can't eat this one.

If you are able to explain to her that after your severe reaction and the whole epi-pen/hospital incident, food looks different to you now. Where other people see something delicious and appealing you see danger and a threat. And while you know she loves you, she is bringing what looks to you like a threat into your house, your safe area....and you really wish she would try a different recipie next time she comes over so you could enjoy her visit instead of being terrified by the percieved threat that other people see as delicious food.

Then thank her for listening to you and being understanding (if in a letter, hopefully in person too, cause hopefully she will be). Like I said this is really hard for people who haven't experienced it to get their brains around.

so i usally end up swollen or vomiting

and in the hospitial myself

I can tell because my mouth and throat start itching and sometimes I get asthma. It is really annoying if I thought it was something safe and then I have to go read through the ingredients (or ask) and try to figure out what caused the reaction. I am sure there are better ways out there than trial and error, but that is how I have found mine. The allergist just confirmed what I told him.