I have to admit that I was going through the same situation like your sister for many years, I thought I was sick all the time, and the more I thought I was sick, the more the symptoms got worse. When I received my diagnosis of Crohns, I started searching the internet, and guess what....I got so depressed because all I read about was horror stories, that finally I was severely depressed. So I admitted to myself that I was facing phsycological problems and I decided to do therapy. I discussed for hours with my phsicologist, and this was the best thing I could do for myself.
But believe me, I didn't pretend I was sick because I needed more attention from my loved ones, I was really torturing myself.
I think the first step for someone who suffers from this is to admitt that he really has a big problem (I mean mentally), and the second step is to ask for professional help.
Now, I feel alot better, still in fear of course for some things, but managing to put logic first of all.