That is very well possible, especially if you are not identical twins, you would still be genetically different from your brother (although more similar than to your next-door neighbour). However, it is not only genetics that drives coeliac disease. Also environmental factors appear to be involved, for instance it has been hypothesised that certain intestinal infections contribute to triggering the disease or make the symptoms more serious. It could very well be that your brother was exposed to one of those environmental factors while you were not, or that you respond differently to these environmental factors because of the genetic differences that still exist.