So i found this somewhere on the amino but basically,, there's a theory (okay spoiler warning if you haven't read KS) When Sangwoo said yoonbum resembled his mother correct? Or well- his ACTIONS really, but Yoonbum's uncle said he looked like his ex. And his dad fell in love with his gf so they got married and had yoonbum. And there's sangwoo where his dad was abusive- yoonbum's uncle WAS abusive.. The theory is- sangwoo and Yoonbum are cousins sharing a mom.