You may never find the underlying cause for when and why the mutation happened. It may have nothing to do at all with your relatives and if it did, no one deliberately passed it along. Sometimes it’s just the luck of the draw when a mutation happens. From my understanding of cancers/disorders, the JAK2 mutation one of the most common.

I didn’t have anyone in my family, either side, who had a blood cancer in the last 4 generations, other than my brother who was doused with agent orange in the army. So there was a blatant cause for his Hodgkins’ lymphoma back in 1969. I was concerned for other forms of cancer though. So I spent most of my adult life eating healthy, avoiding toxins, exercising, no vices…and out of the blue, I developed AML, rapid onset and low odds.
Talk about being clobbered from behind! I had no clue and never even considered a blood cancer. There were 3 random mutations that caused the cancer to develop and no obvious links anywhere. I didn’t even have symptoms until 3 weeks before I was diagnosed. Then admitted to the hospital that day for 5 weeks on a fight for my life. I kept wondering how in the world this happened!! I’ve since learned that it can be something innocuous like a stray gamma ray shooting through the earth or whatever else we may be exposed to on our life journey. All it takes is one little change in a fragment of DNA to change the course into another direction. I will never know what caused my cancer and you most likely will never know the reason behind the mutated JAK2.
Those articles I posted in a previous reply should be informative to answer many of your questions about JAK2. There are many more online from credible sources. Here’s another for you:
Blood-Cancer.com
https://blood-cancer.com/clinical/jak2-mutation
Like I mentioned before about the staggering amount of blood cells produced daily in our bodies that have to replicate with a flawless degree of accuracy, it’s amazing we don’t all have some for of blood cancer. Well, we do all have cancerous cells circulating through our bodies daily. However, our immune system seeks and destroys them. And that’s where the mutations come into play. The defective cells can circumvent our immune system, escaping detection, allowing them to proliferate out of control. Eventually, without treatment to either curtail the production of the cells or knock down the volume, they can overtake our healthy blood and be the end of us.
So, your question, “What is the best course of action?” Talk to your oncologist, listen to their advice, follow their suggested treatment plans. If you’re not comfortable with the diagnosis then get 2nd or 3rd opinion. You’re doing the right thing with compiling questions to ask your doctor.