A man falls in love with a beautiful woman and has sex with her only to learn that she is his sister. A year later, learning she's not really his sister, he sets out to tell her the truth before she marries another man.
Say It Isn't So
Boy meets girl. Boy loves girl. Boy discovers girl may be his sister. (Boy, this could be trouble.) To make matters worse, by the time he discovers "it isn't so," he has precious little time to stop the woman of his dreams from marrying another man.