Larisa Halpert is Jim's sister. She is listed as his emergency contact living at "117 Von Bergen Street" in "The Fight". This is a real street in Old Forge, a town near Scranton, but 117 isn't a valid street number. It is commonly mistaken that the Larisa Halpert who Jim mentions is his mother, but this is incorrect, as Jim's mother's name is Betsy Halpert, and she resided in 383 Linden Ave. until Jim bought the house for him and Pam in Season 5.
In "Niagara", Larisa is mentioned again by Pete and Tom as they ask Michael for assistance in pranking her. In this same episode, a brunette who is assumed to be Larisa is seen in the background sitting with her parents. She also signs the guestbook for their wedding in which she is listed as Jim's sister, writing, "Welcome to the family Pam! We’re so excited to add a new Halpert." Larisa is also seen in Jim and Pam's wedding photo.
More information can be found here.