I don’t think either of them know they’re related though. MC had no idea what happened to his mom, which includes where she went and whether she ever had more kids. I think he also mentioned Sangjin being a cousin by law? From what we’ve seen, I’m pretty sure Sangjin believes his current parents are his biological parents. There’s no indication he has any idea what happened to his bio mom and dad. The father went through great lengths to stage an environment to corrupt and sandbag Sangjin, so he would have wanted things to be airtight so he doesn’t leave the stage that was set for his destruction.
story is just so sad, because his corruption was so subtle and insidious. He’s an adult now and is creating victims of his own, so he needs to take responsibility for his actions, but he really never even had a shot.
And he already got the ultimate revenge on the cheaters. Sangjin’s father was executed before he was even born and the mother was left to starve to death. And she didn’t even love her kids either! He’s not even destroying someone she cherished. He’s just a hateful person who got his revenge too quickly and decided to take it out on the ones who were left, who were just innocent kids. Top tier scum, truly.
So he refused to be into incest and the bitch goes rogue, I don't even know if the so called father has his sanity, if he knew about his son's problem, why influence the young one to get into drugs, then you manipulate the older one into thinking his life is worthless, and worth suicide. Does the fool want to die with the generation after him. Cuz he's killing both his sons, and honestly there's no good enough excuse for this, not even is shabby story of a divorce. He's a useless father, and that's on period!! Uke should man up and face his biggest fear, so his father can die please.