Disclaimer: It's been a while since I played this game, but...
...From what I can remember, the only real netorare in the game is committed by the protagonist. It's a bit of a plot point with the aunt, but I don't remember any scenes where the protagonist gets cheated on. Here are the not-committed-by-the-protagonist moments that I can recall that involve or relate to ntr:
-Near the end of the game, there's a scene where a couple of thugs attack the protag's sister in the park. Nothing actually happens, though, it's just the lead-in to a fight with the thugs, and nothing actually happens to the sister even if you lose the fight (though it does result in a standard game over).
-Technically, early in the game the whole aunt story arc begins when you spy on her cheating on her husband with an unnamed man, record her cheating, and use the recording to blackmail her. The unnamed man never reappears, and the scene takes place before the protagonist starts having sex with the aunt.
-And I haven't played the sequel, but I believe there's a possible bad end in that involving the mother being gangbanged. However, that's an avoidable scene in the sequel, Incest Story 2, which is a separate game.
I don't remember anything about catching someone with your sister, but it has been a while since I played this game.
