The original story was incest. Other relationships change when you turn it on too it's not just the mc. Like Frisha is actually Garon's daughter for example but Emily only tells you this if you have incest turned on. Essentially incest in the game is a hidden switch and modifying the letter flips the switch on. This is a typical work around for patreon so the developer can claim they didn't do it if Patreon ever finds out.
With incest turned on , does Mc become the illegitimate child of the Marcus and the women Marcus was bodyguard of ?
It doesn't makes sense , cause in the letter Marcus very clearly meantions that his love was one sided and he probably didn't even confessed. And that he was envious of the MC's real dad.
I still think the 1st letter makes more sense story wise, cause in the letter Marcus tells us to find a man named "Robert" in Krilic ?
Maybe we'll need to wait .
But it's also intresting that turning incest on makes Emily tell you that Frisha in Geron's child, I don't understand why does this isn't the case for non-incest route , I guess just too fair against Patreon T&Cs.