This would be 'About' and 'Show on Mute' that this topic is about.

I found both to be very compelling stories. In it you have very different relationships between the two. In the first, you have a related brother and sister where the brother is accidentally discovered. Misuzu is in a dominant role and she controls the progression, then plays at acting like he is the dominant partner.

Eventually, he begins to subtly make his feelings apparent, and she relents until he finally asserts it verbally, and in the final moments, sort of crosses a line though she doesn't seem to mind it, more of a shyness 'this might not be the best idea' or 'hey you didn't ask me this time' kinda thing.

Show on the other hand, has a different dynamic. They're stepsiblings (though it doesn't seem to reveal this until things are well underway) and the girl Tae is the one who is discovered in this case.

The way she reacts is differently, being so ashamed to speak (as opposed to crying out like the previous onee-chan) yet clearly she is more than lustful and harbours secret feelings for her step onee-chan.

One pattern in Naoshi's manga is that the girls seem to consistently be given names whereas the male protagonist go unnamed.

Oddly enough, the 'discoverer' in either case seems to be rather happy go lucky and poking fun at the person they walked in on and initiated relations with (which is uniquely different with Misuzu since she took more of a submissive role in the latter half, though one could argue that since she chose this, it is more like a consenting BDSM as compared to Naoshi's later works like Firecracker where, while it's perhaps verbally consenting, it clearly has very conflicted emotions.

-ty