The Man in the Moon 1991 one on netflix

Maureen Trant and her younger sibling Dani share a strong connection, but local boy Court Foster 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.