This one reminds me of Taimanin Yukikaze 1 and 2, in the sense that the only way to avoid the disaster is to avoid going into the disaster area in the first place.
That said, man, if this was an RPG, the MC would have a negative perception score, and the GF would have a negative intelligence score. Though, admittedly, her reasons for giving in to the father's persuasions are better than most sloots in these types of story. Given MC's trauma after the whole thing with his mother, there was a very real chance that even seeing her just get kissed could put an irreparable crack on their relationship.
However, if this were to have a canon ending, it's probably the one where they don't take the offer in the first place, because it's difficult to imagine anyone being stupid enough to bring their stupid hot girlfriend in a house with those types of guys. No one sane could possibly imagine that if they'd do anything else.
Leaving that aside, most of the scenes were pretty decent, but the GF's fall was way too fast. They structured it pretty well, including aphrodisiacs and pavlovian conditioning in the mix... but the process happens too quickly to avoid the fact that she was basically a slut from Day1 that was just waiting for a huge dick to make her cum her brains out all night long.
The corruption is still pretty nice, even if it's too quick. How she goes from loving McCuck-kun the most to being so obsessed with sex and so addicted to big dicks that she either does whatever the father tells her too, or outright asks the MC to fuck-off so she can get fucked 24/7.
And of course McCuck-kun is oblivious to the point of absurdity or pathetic to the point where I'm surprised they didn't go with the old-school suicide end on the side where the brother gets involved.
In any case, not a terrible way to waste a few hour. Or start the New Year lol.