Am I the only person who finds this extremely weird? *Spoiler * I know that it's actually his step-sister, but still they've been family for ~20 years

I remember thinking that same thing during this drama.
I'm currently on episode 13, and i'm absolutely loving it...except for that fact. Kinda weirds me out. 
It's supposed to be weird. It gives it a forbidden love affect, despite that the two girls aren't related. (Or I think they're not related, haven't completed the drama yet! But I doubt they'd take that kind of twist.)
Personally, I think I would've enjoyed the drama more if they weren't identical. To me at least, the incestousness kinda took away from the overall enjoyment I got from this drama. 
Ah, but the main story wouldn't have worked if they weren't identical. Actually the whole story would have fallen apart, unless they had taken it a completely different way. Especially to get the substitute bride.

I think it would have been sweet if instead of falling for a girl who looks like his sister, he would have found another sister in her. Brotherly love! It could have been really sweet. And created plenty of drama. He wants to protect and help both of his sisters!
It is weird. Really weird. 
i think i saw somewhere (cant remember which episode) hongki said that they are half-siblings
Am I the only one that doesn't find it weird? x'D They might look the same but personality-wise they're opposite. If they're opposite he can't possibly look at her the same way as he looked at his sister, even if they're faces are identical. You just come to learn to make the difference in these cases, I presume. 
Yeah, it feels weird at first but when you think about it, he doesn't see Doo Rim as an identical of his sister and actually sees her as a whole different person so I guess you can say he ignored her facial attributes and fell in love with her personality.
It does feel weird if you really analyze it but more so, I think it fits and shows the viewer how much more likable she is than the sister. The sister is cold, calculated and frigid. Whereas, doo rim is so sweet and loving !
I think he loved her like a sister, but since he knew she wasn't he felt like he had to try for more. Or maybe that's because I felt like the script was pushing it too much? It was a rather strange situation... Also I agree with Queen_Makjang, I think the point was to show how endearing Doo Rim was. 
I'm watching Now and I find it totally creepy that he is in love with her. I mean is his sister look alike! That broke the creepy-o-meter!!
But Yi Hyun is not blood related to them. He is not related to Na Doo Rim and he is not blood related Jang Yi Kyung.
He is falling for the girl that looks like his step-sister only because he had feeling for his step-sister.  But once again he is not his real brother at all, and this is made very clear at the beginning of the story that these two siblings are not blood related at all. Maybe I happened to watch a version with better subtitles :)
That's what I kept thinking. I was surprised none of the other characters pointed that out.What an awkward wedding that would look like.But I guess he fell in love with her perfect, Mary Sue personality and not her looks.