This is just my interpretation of it but what I'm almost sure of is that incheol killed the mother of Pyeong-hwa or let her die. Basically years before when they were starting with their political project and needed to evict people his family was disagreeing so the boss told incheol to just get their seal to falsify documents to claim they approved (I think), the mother was about to give the seal to incheal based on a lie he told her but she showed doubt so he killed her. And he just now found out that she's related to pyeonghwa, his superiors knew but didn't tell him.
This is just my interpretation of it but what I'm almost sure of is that incheol killed the mother of Pyeong-hwa or let her die. Basically years before when they were starting with their political project and needed to evict people his family was disagreeing so the boss told incheol to just get their seal to falsify documents to claim they approved (I think), the mother was about to give the seal to incheal based on a lie he told her but she showed doubt so he killed her. And he just now found out that she's related to pyeonghwa, his superiors knew but didn't tell him.