Yes, Jian is Suryeon’s biological son. She never married the man himself but was his mistress instead. Although, Jian is older and a recognized son, the family he is in is obsessed with “bloodline”, so they only recognize him being a mistress’s son. Jitae’s mother was definitely married to the dad and he was born during their marriage so he’s recognized as the“legitimate” son. As for Hajoon, I’m not 100% sure if they are related by blood but yeah, they do really look alike! He’s always been her accomplice since a young age. He was tasked first with poisoning Jitae’s mother. He didn’t seem to do it right so suryeon stepped in and finished her off by stabbing her.
One thing I still don't understand. If Jian is Jitae's half brother, who's the legitimate child? Jitae is younger. Should be Suryeon is the wife? Or Jitae's mom? And what actually relationship between Hajoon and Suryeon? They look alike so much. Like mother and son but partner in crime. Suryeon is crazy enough to trample jitae or jian in order to get power. Wasn't jian his real son? Wth??