I'm no prude, believe you me, and I definitely think that sex can be tasteful, erotic, and truly touching all at the same time... but I am having trouble buying that this game is anything deeper than a well-thought-out and potentially well-executed excuse for incest.
Even that doesn't really bother me, but I do think I would lose any sort of real sympathy I might have for the sister around the time she was asking her brother to cum on her tits. It would cross into the realm of the absurd at that point, which is where most hentai comfortably lies.
Note to gamers: when someone shoots you in the face, they aren't "gay." They are "psychopathic."