Right, Dana and her brother had to sneak around (and sometimes physically hide from, like some sort of silly romcom caper) their father just to hook up. The twins openly live together for plausible economic and emotional reasons, and unless something happens on the trip home they can reasonably pull that off for years while searching for a solution or a change in circumstance, with the majority of the danger coming not from inquisitive parents but from people who are romantically or sexually interested in one of them.
I'd call a willing beard pretty unrealistic for a real-world incestuous relationship (though I most definitely think beards still exist in the real world), but plausible in an adult game that's played by the sexual rules this one has thus far. (By which I mean: Haley has been perfectly fine with, even encouraging of, the MC sleeping with any and all available women.) That said, if the game were to end right now I'm not sure they've yet established anyone well enough to step into the beard role. Lisa and the MC could have an "open relationship" that, to anyone who knows Lisa, would seem odd but in line with prior behavior. Alana could use it as cover for her relationship with Jack, though I don't think that's sustainable over the long term. Adriana's obviously out of the running, Diane would live with them but she'd have no interest in faking commitment, and Sandra wouldn't do it. That said, there's probably enough time to turn Sandra into the sort of character who
would do it, though it would require a pretty radical shift, and I doubt the game's going to spare that much time to transforming Sandra once the MC and Haley are actually moving forward. Maybe this one ends with Haley and the MC newly together and the immediate sequel is "Sandra's Story," in which a career-focused woman who has trouble sustaining emotional relationships embraces her role as a long-distance girlfriend-with-benefits.
As I've said before, while I'd greatly prefer a happy ending for the siblings, I don't really care how the game ends as long as they stick the landing (a feeling that I'm sure isn't shared by most players, who are going to be Haley or bust). Demonstrate that the relationship's impossible over either the short or long term, have them caught and ostracized, have it work out despite what everyone else thinks, make them move to Bhutan, acquire a beard (sexually participatory or otherwise), kill one of them off in a house f...sorry, got carried away there. Just make it plausible and defensible in the context of the story and I'll be satisfied.
Or: a genetics lab hidden deep in the Himalayas manages to clone Haley from a few strands of her braid, after which the clone moves across the country and builds a new life while the original Haley and the MC live the rest of their days in incestuous ball-gagged and leashed bliss...never realizing that the lab has been secretly building a clone army of highly trained Haleys with weaponized braids.
