Say It Isn't So (2001)

Synopsis
Bobby Farrelly and brother Peter Farrelly continue exploring outrageous premises with this comedy directed by their first assistant director J.B. Rogers. In this wacky spin on the boy-meets-girl tale, Gilly Noble (Chris Klein) enters a sexually charged, passionate love affair with girl of his dreams Jo Wingfield (Heather Graham). After a happy time together, they soon realize that they are brother and sister and quickly break it off. Jo begins a new life elsewhere and Gilly is left heartbroken -- until he learns that Jo is not actually his sister; the incest suggestion was in fact a lie begun by a meddlesome third party. As Gilly travels to meet up with his beloved, he finds out that Jo is engaged to be married to another man; he must put a stop to it, even while everyone still believes he is Jo's sibling and is committing an unnatural act. Say It Isn't So also features Sally Field, Orlando Jones, and Farrelly stalwarts Richard Jenkins and Lin Shaye. ~ Jason Clark, All Movie Guide