Say It Isn’t So (2001)

Boy meets girl; boy falls in love (and had wild, non-stop sex) with girl; boy loses girl… when they discover they are brother and sister! But when he learns that he’s the victim of the ultimate case of mistaken identity, the lovesick young man — whom everyone still thinks is after some taboo thrills — must race across the country to stop her from marrying another man.

Tags: crude humor