Alright, so with the post above unambigously describing my own stance on my own situation (incest IRL = Nope)
There is of course the question, why are incest games so popular - when 80% of the forum users voting here are against real incest. They certainly were the #1 tag before Patreon put the hammer down. I can absolutely say that this is a tag i prefer to search for.
You also may want to ask, why for example Game of Thrones and Crusader Kings became so popular - despite or because of their inclusion of explicit sexual incest? Or why people after 2000 years still gossip about Roman Emperor Nero, over rumors of sex between him and his mother, although absolutely nothing proves this claim.
Well, obviously there are a lot of things which are exciting to people, only because they are shunned, taboo, forbidden, illegal.
Like why do so many young people secretly smoke cannabis - even when most of them don't enjoy its effects? Why some people do shoplifting although they could afford to simply buy the goods? Plenty of other things fall into this category. It gives people a kick to do something forbidden without being caught.
So we can assume that at least for the majority of mentally and sexually "normal" people, who reject incest for themselves in real life, still enjoy the topic in the game due to the taboo and the connected excitement. After all, it isn't the player having sex with his own relatives. So the natural aversion is not there, rather a voyeuristic sensation.
"Oh my god! How can they do that! This is illegal! And digusting! And... Kinky!"
And even playing such a game, while not illegal in most countries, would be absolutely shunned and not something you could explain to your wife or other relatives.
Now you could ask - why only incest? Why not gay sex, bestiality, or other shunned / forbidden things?
The answer is quite simple. Incest is actually a biologically "normal" form of sex. Man and woman, sweaty tits, a moist vagina, one penis doing exactly what it was made for. It merely comes with the thrill of the unthinkable. Something you would naturally avoid under all circumstances in your real life. A paradox.
You also could compare this to other fetish elements you would avoid in real life, like anal sex can be really... unsavory. It smells, it can leave a huge mess, it is often painful for the women and bears a significant risk of injury and STD transmission.
But it is one of the most popular porn tags, because it looks great on film. Not something most people could have or see at home. And while vaginal sex always covers the vulva, you get a nice clear view of it during an anal sex scene. Well, so much for observing it.
It also is not only shunned and taboo, it also is a direct health risk for bacteria infection. And abrahamic religions consider it something worth a death penalty. When you think about it, you can attribute these exact same terms to incest. Health risk for the offspring, people hate you for it and may want to murder you. The thrill of the taboo.
Then there is also the point - introducing completely random people in a game is not really relatable. Hi i'm Jenny, you met me on the street, let's have sex. Bad. Boring.
Having a specific scenario does certainly change things. Like a character is your secretary, your babysitter, your maid, your teacher, your neighbor, your in-law? Doesn't need much explanation, the player can immediately relate. And fill any gaps in the writing with his own experiences. This is also how classic porno movies used to work back in the days.
And which scenario is right on top of this list? Indeed, the family. Everyone has a mother, perhaps a sister or cousin. So just a rough description of the family scenario is enough, most players will feel "at home" so to speak...
