I think it's a bit of a stretch to say that's what Oreimo is about, even though the theme and idea underlies the series :-)
There have been far more explicit works. — Aki-Sora and Yosugano Sora feature explicit sibling sex scenes that show everything but the genitals.
I think it's a bit of a stretch to say that's what Oreimo is about, even though the theme and idea underlies the series :-)