It's taboo because the child produced by incest has genetic flaws...reduced IQ to the point of mental retardation, teeth size and extra teeth growing in weird locations, webbed toes and fingers, and really bad mental issues to the point of be a psychopath, and etc.
Sometimes the flaws skip a generation but they show up .
I like playing games like these but please do not try to normalize incest.
Lol. The thing is, that whole genetic flaws thing is actually a way of tabooing incest, you would need several generations of incest, all within close family members to actually produce such genetic flaws, which is probably something that happened in some Egyptian dynasties where the practice of the Pharaoh marrying his own sisters wasn't a rare occurrence.
Incest isn't normal, and the taboo is justified, just not because of genetic reasons (see first paragraph), but simply because if we are wired correctly, we feel familial love or storge or perhaps philia, not eros or romantical/passionate love for family members.
That's what makes it sick or rather unnatural fe. for a dad to lust after his daughters.
Incest often happens when for some reasons family members don't grow up/live together, and so the natural emotional attraction towards family gets misinterpreted into romantic or passionate feelings.
Don't get me wrong, I'm not trying to normalize incest, most incestual acts, are really nothing but sexual abuse and often of minors, no way I would condone that ever, but tbh. I do think there might be better ways than criminalizing half-siblings who only met as adults and fell in love with each other, or that in the US step relationships seem to be treated the same as blood relations, at least when it comes to rules on patreon or societies reaction, or even that while many countries allow marriage between cousins, in others they get treated like freaks or sickos.
But enough of that, when it comes to this type of games, it's most important to always keep in mind they are games.
They are fantasies, a fantasy you have, a kink you like in stories or films etc. it doesn't mean you promote it in real life or would want it in real life, so don't get all righteous about stuff like this.
The sentence you quote sets up the possible taboo relationship, because that is exactly the kink many of us are looking for (in games), it's not an attempt to normalize incest at all.