brother and sister both Polycythemia Vera positive 1 + JAK2 and 1 -JAK2

[myitalia]

I don't know if anyone can answer this but here goes....I was just diagnosed with Polycythemia Vera. This is a slow growing blood cancer. I have struggled with a great deal of health problems and was going crazy trying to figure out what, if any, was wrong with me. I only found out because my brother had 2 strokes a year and a half ago and was diagnosed with Polycythemia vera 8 months ago.  So we both have it.

My question is this.... My brother did not have the gene mutation that at least 80% of the people have with PV. They found it with a bone biopsy. In my case, my doctors knew he was PV positive tested me for the gene mutation after a series of abnormal blood chemitry tests. I came out positive.  

How, in one family did we both come out positive but one had the JAK2 gene mutation and one of us did not? I can not find anything that references this on line and nobody seems to know the answer but to me, its either a medical anomily or there is a simple reason. Thank you, gloria