i am confused about last nights episode i am assuming that brenda and her brother are having sex but did not get the meaning of the books mentioned
Not necessarily, but there is a definite hint of obsession going on. The books were a series that they both dearly loved as kids, about a brother and sister (named Nathaniel and Isabel) who ran away from an orphanage. Since each has got the other’s character name tattooed on their respective butts, we can infer that they strongly identify with the characters and with each other.