This is a YA drama/romance that was most likely made in the 2010s. It was about a girl who has an older sister that disappeared and left clues for her. The night she figured out her sister was missing, she met a guy at a party who apparently knew the sister and they went on an extensive road trip to find her. I read this when i was 15 and have been trying to find it for years.
Lost Classics of Teen Lit: Name That Book! 