Sorry i realize my edit was a bit off. The first lines can be deleted. The paragraph in which begins with "How to get ..... for Gudja" It has nothing to do with the question. sorry
JAK2 most often undergoes somatic mutations, i.e. some of your cells have a healthy DNA, the others not. It's usually not inherited, although rare germline (heritable), mutations are also possible, which can be detected by sequencing. As you know, some people even have PV without a mutation. It's indeed strange that you and your brother acquired the same disease in a different pattern, but as you can see, not impossible. The good news is that properly treated PV has a good prognosis, and if you have children, they can't inherit a somatic mutation. The same holds true for your brother's family, as he has no known mutation at all.