Heh.
I did wonder from the very beginning if his younger step-brother was really a younger step-sister and somehow he didn't know that.
But I've also been wrong before in regard to how someone in a manga was drawn, what sex they were, so I acknowledged to myself that I might be barking up a wrong tree.
Turns out my thinking was actually correct.
If this ever gets updated it will be interesting to see just how that gets played out/explained.