I forgot what chapter too but what I remembered (someone correct me if I’m wrong) his mother and Jaewon had an argument and Seoin happen to witness it welp because they argued in his apartment (its the one that has family photos… his mom being anxious when Seoin takes it down when she visit in some chapters back)…. I think he kept that apartment because he likes that Jaewon died there… during the argument with Jaewon and his mother basically it was about his illness and how tired he is with treatments, being sick in general, being stuck in the hospital and stuff but the mother keeps on I think telling Jaewon to stay strong and then Seoin added fuel to the fire??? By saying something I forgot what and then Jaewon jumped off the balcony and Seoin said something along the lines of you drove him to do that or some shit thats why Seoin was looking at their mother like she the one who killed Jaewon. Since that time Seoin really wanted to cut off ties or at least be separated from his adoptive family as much as he can but he ended getting blamed or taking the blame for killing Jaewon because he knows how much it’ll make their mother go crazy…… again if I’m wrong just correct it… I think it was a flashback around Season1??? That showed where and how Jaewon died????
I don’t remember exactly which chapter revealed how Jaewon died or if it was ever clearly explained at all (please someone share if they know). In today’s chapter, during the Jaewon's funeral flashback… the way Seoin looked at his mother after being punched by Jaewon’s cousin really stood out. The way she avoided eye contact with him and the look on her facd... made it seem like she might have been the one who killed Jaewon, and Seoin is just covering for her and taking all the blame.