Since your white count is the only blood line with a problem, the suggestion seems to be that you have congenital hypoplastic neutropenia. (I think of congenital hypoplasia as adjectives in need of a noun.)
Having a congenital condition would explain why your sister would have it too. Make sure your doctor knows about your sister's results. Although from what I read, most inherited neutropenias are discovered early in life.
We tend to hear more about congenital hypoplastic anemia than about congenital hypoplastic neutropenia. There are a couple of rare genetic conditions that you might read about, but neither is a good match for what you describe.
Kostmann Syndrome is a severe condition; is there a mild form? And
cyclic neutropenia comes and goes week by week, which I assume isn't the case for you.
If you can't fix your genes, having a healthy lifestyle and staying self-aware of your health is the best way to go!