
The person he's in love with is a cousin/distant relative. Her parents died and his parents adopted her because nobody else would. He also has a biological sister who he hates. He calls her sister because that what Koreans do- calling people "hyung" or "noona" meaning "older brother" or "older sister" even if they aren't related.

Spoiler warning
I think from season 1 we get to know that his love for heejoo is actually his craving and desire to be love, and his love is not romantically, it is more like emotional dependency which is easily confused with love, because they both usually come with intense feelings around another person. I think in next season when yuri finally fall in love with sunwoo, he will be able to tell the difference between his love for heejoo and his love for sunwoo

i mean you're definitely right, but right now he thinks he's in love with her, he's suffering because she married someone else, he's been raised as her little brother, she sees him as her little brother too and she's his actual relative to boot, not just adopted. and he doesn't seem to be weirded out by the fact that it's not normal to have those kinds of feelings for a sister or a cousin. it feels like the author is normalizing it in a way and i can't help but be skeeved out by the whole thing. it's just a writing choice i'm never gonna be able to understand even if i believe what you're saying is right, you know?

I personally think the author isn't normalize yuri feelings for heejoo. Because both in fyl and fos, the author subtlety describe yuri's feelings as something that better left unsaid or kinda forbidden and different from sunwoo's feeling to gu jungho which is unrequited love. I don't know let we see as the story progress. And yes i agree i think deep down below his consciousness yuri is interested in sunwoo.
So i'm in the second chapter but i'm confused as hell
could anyone tell me if Yuri LOVE!!! HIS SISTER??
cuz if this is the case i will drop it
is she his sister or a relative this is not clear at all..