The Turn of the Key - Ruth Ware

Okay, it´s prediction time:

 

"Rihannon, listen, I can promise you this - this is absolutely cast iron. I swear on - on my grave, I am never, ever going to sleep with your father."

Because.

It was on the tip of my tongue.

I am never, never going to sleep with your father because -

 

 

 

- he is also my father.

 

...

 

[Source]

(show spoiler)

 

Thank god I´m almost done with this book.