Don't get me wrong, I love this game and I love the main plot, and their relationship was extremely well introduced, as to why both are so connected to each other, but step-siblings isn't incest. At least to the point I've played it.
I don't want to start a crusade towards the dev btw, just stating that your answer doesn't apply in this case, even if I love the story and how the connection to each other was introduced. It still has a strong taboo feel, which I still appreciate when I'm playing a game, specially if well written, but it isn't incest.