There are probably tons of people who've read this manga and surely all of them has a different interpretation of it's story and ending.

In my opinion, this manga is a conversation between our two main characters, Se-Wa and Se-Joon.
In the beggining, we hear the story of a man, who was loved by all, but could not love anyone, until he met this girl. Even though the girls loves him too, she cannot won't leave her husband for him, and thus he will be alone in despair and grief for the rest of his life.
The storyteller asks the main question of our story, which it builds upon:
"Would you choose to be loved by everyone else or would you choose to be loved by one person?"

For 26 chapters, we can see a life where he chooses one of the options. And at the end of the story, when you think everything has ended, we hear the conversation again, now with the characters, and we can get to know which choice he/she would make.

As for the story(9.5):
It is a full, round story even without these conversations at the beggining and the end.
Don't except too much laughter, it is truly a tragedy. A psychological, romantic tragedy. It has its own twists you can't even imagine. I had several ideas, one worse than the other, but I was totally surprised at the end, although looking back, everything seemed so obvious. The little movements, short sentences all led to this wonderful ending.

Characters(10):
They are a "twist" too, in a way. You can't be sure what their personalities are, until you have read all of it. They're not shallow, but really mysterious, although you might think you know their way of doing things after reading the first two chaps. Don't be fooled :) The few supporting chars are great too without too much cliche.

Art(10):
Wonderful. That's all. Really. You can find the usual "manhwa marks" in it, but it's actually unique, and just gorgeous.

Overall, it was worth reading. I almost cried and laughed at the same time when it ended. And just because it was over and I couldn't read more. This mangaka is epic. You should defenitely try it out too, even if you are disgusted by incest. The focus is not on siblings loving each other, but two strange(insane because of love and other stuff...) characters, who love each other in a unique way and have their own, crazy ways of showing their emotions.

(sorry for my english, i'm not native)