The Man in the Moon (1991)

Maureen Trant (Emily Warfield) and her younger sibling Dani (Reese Witherspoon) share a strong connection, but local boy Court Foster (Jason London) threatens to throw their bond off balance. Dani and Court meet first and have a flirtatious rapport -- but when he meets Maureen, he falls hard and they begin a passionate affair. The new couple try to keep their love hidden from Dani, but she soon learns the truth, disavowing her sister. But a heartbreaking accident later reunites the girls.