Like his Mentor, Dr. Benton, Dr. John Carter also misses his graduation.
At the beginning of the episode, right after the intro, Carter and Dr Benton are coming down the stairs and Carter mentions that everyone is coming to his graduation, including his sister. At no other time in the entire series is a sister mentioned. Only his brother who died of cancer when they were young. The writers decided that a sister would not fit into Carter's future storyline so they magically never mentioned it again. The sister was mentioned in season one around Thanksgiving.
Marg Helgenberger's last appearance on ER.
At graduation we learn that John's full name is John Truman Carter.