beta
Theory time! Ha-yeon is the kid the mom had either from a random affair or from the person she actually loves, and he looks like her and means more to her and that's why she wants to actually have him in her life, and Hajin is the kid she had to have with the man she was forced to marry (or Hajin is a random kid they found who looks enough like the mom and dad, because the mom refused to have sex with the dad or was unable to have kids). That's why Ha-yeon is all like "You stole from me"