Okay, someone just told me that apparently all known Skywalkers, except for Shmi, are Force-sensitive.
Though this is entirely theoretic, I would assume that the chance of a normal family having a Force-sensitive child is about one or two in ten thousand.
In an established, known bloodline (aka the Skywalker dynasty, the Gallia and Allie families, or the Kenobi or Daemes families, in my imagination universe), it would likely be around one or two in a hundred.
If one parent was Force-sensitive, then the chances might be as high as twenty-five to fifty percent.
If both parents were, the chances are likely to be around fifty to seventy-five percent.
So, why are there all these Force-sensitive Skywalker kids?
I would assume that it’s due to the size of their families; they never have the fourth child who isn’t Force-sensitive.