Oh good grief!!! How awful for a family member to inflict such emotional pain, not even realizing that by doing so they're also causing you physical pain (I'm guessing all that stress didn't help your symptoms at all)!
My family honestly just didn't get what was wrong with me or how sick I was, because I never saw any of them when I was sick. They didn't really come to visit me, and I would rest up for days before seeing them. If I was having trouble while seeing them, I'd have to suck it up or go lie down, but they never really SAW me having trouble. (Unlike you, I'm a leaker, so it isn't as obvious.)
I'm guessing this person has never seen you in anaphylaxis. I can't imagine anyone saying it's all in your head if they had seen that. So on one hand, I would cut them a little slack because they're entirely ignorant. On the other hand, they clearly lack any form of empathy or tact.
It wasn't until I finally got a diagnosis (eight years into my most recent horrors) and sent Dr. Afrin's very long, descriptive email to my father that he FINALLY seemed to get just how debilitating this illness is and how very difficult it is to diagnose and treat it. I feel very vindicated, but in some ways, it's too little, too late in that I know everyone harbored such doubts about me.
I'm afraid that some of your family just aren't going to believe you until they actually see you in crisis or you get a diagnosis (backed up by labwork). It's very hard for some people to accept without seeing it with their own eyes. You need to concentrate on keeping yourself healthy and sane, and let them think what they want. You absolutely have my sympathies, though, and I'm sure most people on this board can relate to this.
We believe you.
-Kat