Here's a synopsis that matches your description:
* The Summer Man follows a young woman named Rachel who takes a summer job at a luxurious country club.
* She becomes entangled with a wealthy and enigmatic man named David, who is rumored to have been involved in the mysterious death of her older sister years earlier.
* As Rachel digs deeper into the past, she uncovers secrets and lies that threaten her own life.
Let me know if this is the movie you were thinking of!