Some thoughts:
1) Perhaps the girl who survived the double suicide was Reiji's mum? And the fact that Esemori used her incident as a plot for his book might have caused feelings of disdain towards him, hence why she said that he was a horrible person.
2) Esemori is probably gay..? I first thought about the possibility when Nagi said that nothing happened between them despite being married. He's still a complete mystery.
3) There's no way Esemori is Reiji's and Nagi's dad, Nagi is also not Reiji's sister ya'll.... Incest marriage is illegal, and the theory itself doesn't make sense either.