Not sure if it was said in a comment or in the story, but I think Damiens parents were deceived (not told they are related) by their parents until it was to late and that this was a reason why Damien's dad killed them. Like I said not sure if I'm right.
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Actually there is this other part of the story that each man marries two women. One who is his sister and the other is a cousin. I guess. Something like that. So Damiens dad did not know that his wife and him are related cuz they look different. Until he found out his own family actually orchestrated the whole incident and he was so disgusted by the whole thing. And when the family wanted to kill our male lead, he killed everyone and then gave everything to the male lead and killed himself. Omg this story is so intense.
But didnt they say that a boy in the family can only be born if a male and his sister/cousin/someone they are related to have intercourse. And a girl can be born from a male and someone outside the family. So does that mean that Damien’s mom is prob his aunt aswell