Seems like it is a part of the game. If it's not, who are the "Mom/Sisters" ? his roomies and landlord? the Dev can say it doesn't have incest, but it does. I am not 100% sure and am only guessing. It just won't make much sense if it isn't his Mom/Sisters.
But if the Dev didn't add "define the relationships" or "name each character" (cause Patreon) I don't think I'll be buying this on Steam. Getting tired of these games needing a patch while some others don't need it. If it needs a patch, no buy from me.
The game has incest and the relations are Mother/Son plus two sisters I can assure you on that, kisskiss had this in mind when creating the game, the problem is due to:
Patreon (Paypal A.K.A. Visa) banned incest ( above other controversial contents) since 2017 so if kisskiss want to sell the game on Patreon and Steam, incest must disappear from the table, simple as that.
I agree with you it's a stupid thing to do to an adult product, but who allow the money to flow it's not you nor me, it's Visa ( for Patreon and Steam) and they decreed this for adult authors or producer of such contents, so it's either landlady/ward/ etc etc or else they will not gain money, or worse they will not be allowed to go on these platforms.
For now on Steam it's rather sparse, hence you still find some titles that don't requires patches, but this is the future and the more time will progress, the more adult games and other media producers will be forced to self-censor themselves, if they still want to be allowed to continue selling their contents.
On Patreon the censorship is starting to be more strict, sadly bigots are winning in this current historical moment and there's nothing you nor I we can do at the moment.
Now since these games aren't made for fun, but they are made investing money, to survive they need money and if whoever controls the money tap tells you no incest, you have to remove incest, period.
Once again I'm as tired as you of this hypocrisy, but since I want to play these games I will bite the bullet since for now there's nothing I can do.
Let's hope that in the future these morons will regain their sanity and allow again without restrictions adult contents to be sold without issues.