Imagine they grew in orphanage. They are not blood related, and yet, everyone grew together like childhood friends or siblings. Between some of them, some actually fell in love because they never regard each other as blood relatives. In that sense, Eugene never really saw Hari as Arina (his actual sibling). Eugene just treated her as his junior, but not blood relative. Given the orphanage concept, it was actually understandable.
You completely missed the point that was made. The fl literally LOOKS like his dead sister. Thats the main reason she was adopted. If you saw someone who looked exactly like your sibling, would you actually be able to look at them like theyre marriage material.. Its weird because you’ll still be seeing your sibling in them.

Honestly I stopped reading after I figured out the ML was the step brother. And I'm still weirded out to this day because like?? Are we forgetting that she was even part of the family because SHE LOOKED LIKE THEIR DEAD DAUGHTER????? H E L L O ? ? AND NOW THEY'RE MARRIED??? I KNOW THEY'RE NOT ACTUALLY RELATED BUT STILL