Would incest make the story any better? No, so why should the dev even bother? Not every game here includes incest, if you can't live without it, there are enough alternatives. The vast majority of games with incest patch were planned as incest games from the start. By that I mean the tons of Landlady / Roommate games where the names are simply exchanged.
Our Nicole is insecure enough as it is, she would probably go crazy if she found out that she had a sexual relationship with her stepson and that he also had sex with his stepsister (her daughter).