4 human male, I believe you forgot the Emperor.
Also the Leah being force sensitive was eluded to.
The brother and sister outside of the trained count have to refer to Luke and Leia (Luke isn't trained at the beginning of the trilogy, but is at the end), so 4 human males (Luke, Vader, Kenobi, and Palpatine) in the trained count would double-count Luke.
STOP, now a related thought about why one of them is unlikely to reproduce due to the limited female characters in the movie. "On top of that we have a brother and a sister."
But I can see how your reading of it would make sense, since it removes the potential count error I read it as having.