Ch. 13 reveals that he'd known they were vamps for a while. Looks like he figured it out soon after he started living with them. We're not shown exactly when though.
now I know that Sangwoo didn't kill either of his parents, his mother killed his father, and the mother killed herself by pushing her self onto it when it was in Sangwoo's hands... MY LIFE HAS BEEN MESSED UP READING THIS MANHWA FAM!! Σ(っ°Д °;)っ
Why does this go on a hiatus every month