Oh, I know. It's just amusing, that's all.
I'd think mother-son is (it certainly is on erotic fiction sites), and unfortunately that's the one kind I don't particularly enjoy reading. Or, more relevantly for this site, playing. That said, I go ahead and apply the incest patches anyway, because otherwise what's the point?
I don't really have a personal interest in which way they go as long as they stick the landing, so to speak. Assuming Haley and her brother are going to end up with each other (and "we're in love but the societal pressure is too much to deal with, so unfortunately we can't be together" is a completely valid, realistic, and narratively coherent way of ending this story, assuming the devs want to make half their audience terribly sad and the rest hilariously furious), there are really only three options: live in terrified secrecy, declare themselves openly, or grow a beard. Now, in the real world that third option has something like a .000001% chance of being the actual solution, but this is an erotic game and the rules are out the window, especially as the writer's played that card before.
That said, the first two are devilishly hard to pull off. Yes, for a while the twins can just go to work as siblings, hang out with friends like normal people, and keep the Daddy/leash-and-ball-gag stuff private, but eventually they're going to face questions. "Why don't you two ever date anyone?" and so forth, especially from their parents. There's a potential story in the extremes to which someone will go to hide their relationship, but I don't think this has ever been that game. Whereas the second option just assumes that everyone's going to be totally cool with incest. Again, porn game logic can apply and incest can be just another thing that everyone's doing and completely acceptable to the universe at large, but this isn't that sort of game either. It's a writing challenge, to be sure.
A beard's kinda the easy way out — the blank chip in Scrabble, the Joker in the deck — but it's also the easy way to replace an emotionally satisfying ending with a sexually titillating ending if it's not completely justified in the text . For a beard to work, we have to believe that 1) both Haley and the MC are cool with him or her, and why; 2) why the beard would be up for it in the first place; and 3) what the emotional and sexual rules are. "Diane, move in with us." "OK." isn't a story, or at least it's not an interesting or good story. I'm not afraid they'll do something like that, because the writing has yet to let me down — if anything, the opposite — and I'm really interested to see how they make this work but emotionally uninvolved in exactly how they do it. As long as it's good.