The chapters were initially missing half of the chapters and they were just uploaded. You should reread chapters 36-the current chapter because the part 2s of those are now available when it was initially just the first half of the chapters.
I’ll tell you what I remember though. She has the power to seal yokai. She sealed the yokai that was possessing Yako’s mother, Tokiko, inside her which is why now the mother has the x on her forehead. The guys sister doesn’t really have anything to do with her except for the fact that they lived very similar lives when younger with the abuse she faced by her family. He wised that his sister could’ve been saved like the female lead was when she was chosen to marry Yako. I think that the yokai that was sealed within herself is unknown. She doesn’t remember what happened to her after the great monkey yokai took her when she left the barrier as a child, just that when she woke up and reappeared she had a yokai scent to her and the x on her forehead.
Is there a chapter missing because I slot the plot right when he wanted to help him with her power….like wtf is going on with her power ? What is her power? How are they related? What does the guy’s sister have to do with the female lead? What demon was sealed on her forehead?