These are really good perspectives and I suppose you are correct in my not understanding the step relationship. He would likely view them as his daughters considering the time they have been in that type of relationship, but his "dream" sequences repeatedly mention/affirm that he is interested in them in a sexual way. I agree with these and other comments that it is getting a bit repetitive and there should be some corruption influences happening with his behavior/thoughts. I know that is in the works based on the dreams and inner monologue, so I will be patient.
I was actually unaware of the legality with the step-daughter in the UK/other locations (and didn't think about the adoption part, which may make it illegal everywhere) so that does certainly help to explain those thoughts.
Perhaps using the word "realistic" was inappropriate in my comment. Very few (if any) of the incest type games (I know this is not incest...) are actually realistic. They are a fantasy and the taboo is part of what makes them interesting. Apologies if any of my random thoughts offended, that was certainly not my intention. I actually really like this game and think the renders are absolutely beautiful (which is why I created a gallery mod). I have tons of respect for the creator and look forward to seeing how the story progresses.