Lone Star
At the end of Lone Star, Sam (Chris Cooper) and Pilar (Elizabeth Peña) finally learn the truth about why Buddy (Matthew McConaughey) and Mercedes (Miriam Colón) were so vehemently opposed to their relationship: the two were secretly half-siblings all along. Despite this, since Pilar cannot bear any more children, the two decide to carry on with their relationship. Yuck.