Pola X (1999)

Pierre, a young man of privilege whose anonymously-published novel is a hit and who is about to marry his beautiful blonde cousin Lucie, abandons everything when a brunette vagrant tells him her secret late one night in the woods: that she is his own sister Isabelle, abandoned by their father. Pierre breaks up with Lucie and leaves his doting mother to head for Paris with Isabelle, intent on knowing the dark side of human nature. He begins a novel, sending chapters to his publisher under a pseudonym and his relationship with Isabelle moves beyond the fraternal. In the winter, frail Lucie comes to live with them and family jealousies mount. Pierre might have discovered despair instead of truth.