Someone touched on this awhile back, but I think almost any believable incest plot can be achieved by creating interesting, sympathetic characters. I don't prefer incest stories/games necessarily, but I do enjoy them. With two normal adults, the only real barrier or challenge to sex is one person not wanting it (at first), so they need to be seduced. But good incest plots generally require levels of sexual curiosity, self-denial, taboo, subtle indirect seduction, and ultimately overcoming the natural barriers to a sexual relationship because the love (and lust!) is just too strong. In other words, "I shouldn't be doing this, but I just can't help it because...." If you can make the "because" believable, you have a believable incest plot.
For anyone developing an incest-themed game, I suggest reading some of the top-rated titles in the incest category at Literotica.com. "Words on Skin" for example might not be "realistic" in the sense of creating a plot where a brother and sister would have sex, but it damn sure creates a world with characters that are believable, which makes the story believable. Read a bunch of those stories to get a feel for what works and what doesn't work. Identify what highly rated erotic incest stories have in common, and then most importantly put your personal stamp on it.
A realistic incest-themed plot would almost certainly be dark and deeply psychological. But a believable incest-themed plot just needs fleshed out characters so the reader/player understands why they are so attractive, and attracted to each other, that they are willing to cross that barrier.