Not a love triangle in the technical sense, Atonement does see what happens when sisterly jealousy gets out of hand. The lives of lovers Cecilia Tallis and Robbie Turner are torn apart due to a lie told by Cecilia's younger sister, Briony. The three must then deal with the aftermath, with Cecilia and Robbie getting a second chance at love when their paths cross during WWII.
Watch Atonement on Netflix.