Say It Isn't So

Jo and Gilly date. They find out they're brother and sister. Jo moves away. Gilly finds out that he's not Jo's brother and that Jo's getting married. Can he stop the wedding in time?
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.