Charlotte Wade was Lexie's twin and sister that died during childbirth. Her parents and Lexie used to visit her grave often, however Lexie's parents had stopped going a few years prior to the Pilot. Her grave is located in The Gates Cemetery and her marker says her name, date of birth/death and "Not Forgotten". (Identity Crisis)
Lexie takes Charlie Monohan to visit her, and on finding out that it was her sister, he sets a flower on her grave and says that it was nice to meet her. Lexie reveals that she hasn't taken anyone else to see Charlotte before him. (Identity Crisis)
It's currently unknown if Charlotte would have been a werewolf like her sister or if just a human.