
**spoilers????***
tl;dr at bottom if you don't want to read a big dumb wall of text
Honestly it's way to early to make assumptions, but this is from the raws - bear in mind I don't speak Korean so this is just gleaned from my understanding of the art.
I think the reason behind Yuri's love for his cousin stems from the fact that she has been the only person in his life (so far) to have ever shown him affection and care. The family is shown to be pretty fucking cold to each other - the parents don't give a shit about any of the children except for the adopted daughter (Heeju?? man I suck at names I'm sorry), Yuri's eldest sister is an ass to him, and the other children at school shun Yuri for having an adopted sister (children are fucking mean, who knew). Yuri's cousin remained the only person to have ever been kind to him, and because Yuri's metric for affection is the emotional neglect shown to him by the rest of his family/peers, Heeju's care probably felt like a beacon of light to Yuri. Thus, from this familial love, Yuri developed a puerile crush that morphed into an unhealthy adoration for his cousin.
From this theory though, we can probably see how Jiri and Yuri's relationship will progress. Jiri will become the other person in Yuri's life to show him the affection he missed out on in childhood/adolescence/early adulthood. Jiri has remained so fucking kind even though Yuri has done nothing in this story (not counting FYL) but trample over Jiri's emotions. Jiri's nearly unconditional care will probably give Yuri another anchor to grasp onto in light of him finally letting go of his obsession (is that word too strong?? idk) of his cousin, and Yuri will likely slowly fall in love with Jiri as a result. Of course, Jiri still has his own issues and internal conflicts and I don't think he can stay as a mere pillar of support for Yuri to lean on - I'm pretty sure their relationship will be one of mutual care and healing (that sounds really stupid I'm so sorry lmao).
I could be wrong with this, since I, yknow, don't speak nor read Korean, but it seems to be what was depicted in the raws. Also, I just kinda skimmed through the flashback portions of the manhwa in the raws because I couldn't be bothered to dwell on them. I still dislike Yuri and think he's an asshole - this isn't a defense of him and I think his actions are pretty despicable, this post just provides a p o s s i b l e r e a s o n as to how his childhood has affected him as an adult.
tl;dr: Yuri is emotionally constipated and since his cousin was the sole person to show him care during his childhood, he developed an unhealthy crush on her. Jiri's unconditional caring nature will probably be the root of his and Yuri's relationship as the story progresses.

I just read the raws and dammit, yuri is still a gaping asshole for like the next five chapters holy shit. Imma be honest, I really don’t care about how he feels about his sister. I genuinely don’t give a shit for the flashbacks. I honestly care way more about Jiri because at least Jiri is compassionate to others even when he’s suffering alone. Jesus Christ this man is getting fucking slapped around by Yuri just because the fucker needs an emotional outlet and Jiri’s nice enough to let him do so. Christ, I wish I could go into the manhwa and just give Jiri a hug because he fucking needs one

honestly, that part *obv spoilers* when Skyler reminisces and realizes that his mom had tried to kill him quietly by the portside when he was younger and Yeonduk had inadvertently saved him, really fucked with me. idk why, but imagine looking back on your life, realizing that the man you looked up to as a father was a dead-beat piece of shit, that your mother had wanted you dead for the majority of your life, and that the friend you'd hated for like ten years actually saved you multiple times. damn, dude
how did he start walking right after he woke up??? mf was is a /9 year coma/// how'd he jUST GET UP AND WA L K???