I've seen that happen in some mob documentaries where the daughter never understood her dad was a mob boss until as adult the police finally caught him.
You know that there's their lawyer behind the camera, remembering them every two minutes before the filming starts that if they say something else, they can be held accomplice for some of their father actions, right ?
Every close family member saying in those documentaries that they where totally clueless are just saving their own ass. Face to possible prosecution, but also face to their future life. Because they have a life, and for the rest of it, that can easily be 40 years or more, they'll have neighbors, co-workers, and expect to have some friends. They just can't say, "yes, I knew", and expect those future persons to not be at least suspicious around them. They'll have witnessed some odd behaviors, because they also don't want to looks totally stupid, but they never thought about them with malice and have a perfectly legit explanation for everything.
i've also seen similar thing with serial killers whose family often sensed SOME things were not okay to ask about.
What isn't the same thing.
Being a serial killer isn't a full time occupation. Most surely think about it 24/24, but it's not their day basis occupation. Same can be said about hitmen, who have a cover job, and use it as excuse when they need to be absent for few days due to a contract.
Anyway, here again, no one will recognize that (s)he knew everything, it would be a judicial and social suicide.
all that said, it seems your issue is more with bad writing than incest patches.
I only addressed the patches to say that I rarely use them nowadays... And I explicitly said that the issue is with the story context. A game like
Triple Ex, by example, would be ridiculous without incest, since among the twentish girls the MC can have sex with, only two aren't family members. But there's also games that have the opposite issue, and stories that looks totally ridiculous when put in a family context ; but because of this ridicule, no names cross my mind right now.
the job #1 of the writer is selling the emotional truth of the story, and if he fails to convince you that an 18yo son suddenly wakes up wanting to fuck his family, that's a writing problem.
No, it's not a writing problem, but a premise problem.
As stupid as the trend was, the succubus contract was far more believable than incestuous lust waiting for the MC to be of legal age, then growing in one night. Will the resolution should be like a dam breaking, and isn't, this is more a detail than a flaw ; it's less believable, but excusable due to the story. But incest is a feeling that grow slowly, and the MC isn't different from the family members he pursue, he started by denying his lust, then fought against it, before finally giving up to it. And not respecting this is not being a bad writer, but not knowing what you'll talk about.
A game like
My New Memories is believable because the MC have sex with his sister-in-law since officially 18 years (because he'll also have sex with the daughter they had together), and with one of his sisters since longer than that. He never thought, or dared, to also have sex with his daughters for whatever reason, but he haven't woke up one day, discovering that he had such tendency, doing it precisely when the story starts.
so many easy ways to frame the situation right, like adding one sentence of mc remembering getting a boner for sister on a summer camp when he first saw her in a bikini.
Oh god, it's even more lame than the succubus contract...
It's confusing a physiological and uncontrollable reaction, with the lust that incest is. Most guys between 13yo and 16yo had at least one boner for their sister/cousin in bikini, especially if they are near to their age. What doesn't mean that they had incestuous thoughts, nor that they'll have incestuous lust in the future. It just mean that they are functioning males and where full of hormones due to their puberty.
just few words can put the idea into reader's brain that mc had urges for a long time, but only now realized it.
But he waited to be of legal age to suddenly decide to act... Please, give us back the succubus...