Jokes aside, Idk how I feel about Shani being added to the harem ngl. I was feeling icky about the younger sister too but her thottery ways convinced me otherwise. All in all plenty of decent content and the setting is well established. I feel like we need a lot more character development with the core cast though, barring Shani.
Apparently incest is pretty normal in that society, so no reason to feel icky.
In my opinion, there are clear differences between the two daughters in terms of their characters.
The older one is adventurous and independent. The younger one is meek and affectionate.
I think that the MC accepts their respective characters and raised them in a way that he feels will benefit both the MC, themselves and the household in general. The older daughter has the rare qualities needed to become a stalker like her father and will therefore be able to take over the household in due course. The younger one doesn't have these qualities and will remain dependent on someone else for her entire life. I think that this someone else may even become the older sister, depending possibly also on how the story progresses.
I think the decision of the MC refraining to have sex with the older daughter is a clear sign that he respects her as an independent person. An independent person is something the younger daughter will never be, because that's not in her character.
Possibly the MC knows that he is abusing the younger daughter but is convinced that it will not hurt her because of who she is. However the MC is probably afraid of abusing the older daughter because of fear that she may change and turn out like the younger one. In the last scene, it became clear, however, that the older daughter felt left out and wants to have sex with her father and that this will not change her, but actually may make her stronger.
I am really curious to see how the dynamic between the sisters evolves in future updates.