i mean i'm just assuming he is kissing his brother bcs the panel of his brother was next to it. maybe he kissed another guy bc some bet with his brother (i feel his brother was manipulative or had an obsession, idk. But now it's either hospitalised?? or dead??? anyways he's not around and maybe won't be), Y saw it and got some confrontation with S's brother bc of it. The confrontation: maybe S's brother felt S was leaving him behind bc of Y, and a way to get back at him is S kissing another guy? Anyways thank you for coming to ted talk full of ramble, bcs there is absolutely the possibility that it is incest and not another guy
The comment or above said it's most likely is incest. I'm gonna assume the brother is jealous and kissed his brother (Seo-In) because he is in love with his brother. In the scene where the Dr came by to drop off food she said to Seo-In: "I know I maybe your brother's doctor but I've known you forever" and to "make up with his mom" or something to that effect. So in the second read I conjectured that to mean the brother has some mental health issues or is just really incestuous. Seo-In either went with it or was actually also in on it w the incest. Idk. I'm just guessing based on how SI said his mom is just worrying about what she's lost so at this point we're just assuming it's solely the brother but still...and why did YW get blamed for it??? Did he spill the beans??? And if not, were they just gonna continue being like that???

wait so S was kissing his brother in the class? i say this bcs of the panels in chapter 2 (2 guys with a mole below his mouth) and chapter 6 (the pic in S's house with these two "same" guys)