I'm not sure that's correct. That would seem to presume that there never would be incest anywhere ever. True, it's fairly rare unless it's exploitative, coercive, abusive, or similar. But it does happen. In cases like this game, I just accept (i.e., willingly suspend my disbelief) that this is one of those rare instances.
I'm neutral on incest, to be clear. It's not a fetish / kink of mine, but I also don't dislike it. But, my suspension of disbelief is really pushed when people who have no connection other than compatible body parts decide to jump each other aggressively after only saying hello.
I know not everyone thinks the same as I do, and I'm OK with that. Hopefully everyone else can be OK that I see things differently.