I think he's a perverted and creepy  f***** 

Basically he falls in love with his sister doesn't matter that she's not he's real one let's see who looks exactly like her how can he think of kissing her having sex with her it's totally weird something's wrong with him.. and also it doesn't make sense that no one in the drama thinks that his feelings are weird they all act like it's completely fine for me that's the weirdest part about it.. :)  

 Apple Eye:

I don't get all this anxiety about supposed incest. In episode 3 the mother of Yi Kyung accuses Yi Hyun of deliberately postponing the search for her daughter: 'Wouldn't you look for her already  if she was your biological sister?!' So they both know Yi Kyung isn't his sister, but somehow they're keeping up appearances. Yi Hun  can fall in love with Yi Kyung but he's hindered by family secrets.

MAY I KNOW THE NAME OF THE DRAMA?