Didn't seo-in know that his brother beat up yaewon?

No he has no idea, and he's also unaware his parents paid off Yeongwoon's father. I think YW just realized Seoin was unaware, and is now keeping quiet about it. Crashout will happen for sure once he finds out

pretty sure the mother implied that seoin is not to be told anything about the beating and payoff when yw asked to say goodbye to him and she said no, no goodbyes. back then (when they were kids) it seemed to be that YW was promising that he won't tell seoin anything, just wants to say goodbye, so 10 years later he'd still be under the impression that the secret was kept away from seoin. Personally I think he, YW, still thinks seoin had a crush on his own stepbrother and back then didn't want to know who he would choose between the friend and fwb and the stepbrother and crush, and now especially since the brother is dead, he doesn't want to spoil the brother's image on seoin's eyes, since everything is done and over with. And yes, crashout incoming, mother better watch out

I forgot about that scene with the mom smh Can't wait to see the parents downfall