Everything you need to understand or teach Two Sisters by Gore Vidal.
Two Sisters is a series of permutations on the theme that one cannot recapture the lost loves of youth because they were not the loves one thought them to be. The plot, especially in light of the complicated technique, is ever so slight, but the numerous pieces of material for a plot (including additional autobiographical elements in Eric's screenplay) are brought together in the last few pages of the book as the character Vidal learns almost inadvertently that he is not, after all, the father of Erika's child.
The son Vidal had lacked the courage to acknowledge is revealed as the fruit of Eric's incestuous liaison with his sister. The tenuous thread tying Vidal to an unseen and unacknowledged replication of himself snaps, and no connection remains between him and either the Erika or the Eric of twenty years earlier.