If I am correct the story goes like this: Min Siwon was the illegitimate child of his currently married father. The fathers real legitimate son got il and they could not find a cure so, he brought his illegitimate son to replace his wife's son to avoid any scandal. ( cuz he's some big shot executive or what not). Anyway it seems he needs an heire to all his fortun and his only heir was sick so the only alternative is to pretend his son (out of wedlock) is his heire while he's still young and no one can tell. Essentially "out with the sick and old in with the healthy and new". Sick right???
Thank you
Man I'm so confused now ╥﹏╥