if this is really going down the lovers route, at least they made it believable, they are not only stepsibling, they also didn't spend much time as supposed siblings, so I would be okay with it, even if I really have no idea what exactly is so fascinating at incest that you see so often either a very unhealthy obsession about a blood related sibling or like here lovers who grew up as stepsiblings (sometimes they even grow up as real siblings and get their not-relation known to a later date)
is it so difficult to write normal sibling relationships? you generally like them, sometimes you could kill them, providing help, advice and comfort to each other, a real family...