Sandy never refers to Daria as Quinn's sister. Instead she refers to her as Quinn's "cousin or whatever", "that girl that lives with you or whatever" or "that exchange student that stays with you or whatever" to Quinn.
During the end credits of "Is It College Yet?" the futures of the characters are revealed. The three members of Mystic Spiral become gospel singers, Andrea the goth becomes a model, Quinn becomes a CEO, Tom becomes elderly needing the help of a carer, Jodie becomes the owner of "Landon Towers", Sandy works at an escort agency, Jake and Helen live in a cabin in the country and live on Valium, Mack and Kevin become owners of their own ice cream company, Lindy and Alison live on a farm with three children, Tiffany becomes a fortune teller, Mr. DeMartino becomes a mail man, Britanny becomes an actress, Ms. Barch becomes a reverend, Joey, Jeffy and Jamie becomes MTV broadcasters, Trent becomes a judge, Mr. O'Neil becomes a theatre actor, Stacey becomes a race car driver, Upchuck becomes a millionaire with women on his arms and Daria and Jane becomes talk show hosts.
The last episode of the show's half-hour series, the fifth season's Daria: Boxing Daria (2001), was written to close out the show. After the creators were offered by MTV an option for another half-season (6-7 episodes) and fans begged for more, co-creator Glenn Eichler decided instead to make a 90-minute TV-movie, Daria in 'Is It College Yet?' (2002), as the official finale.
There is a small recurring joke involving Trent writing song lyrics as he's doing something else (such as driving). He always hits a snag, needing a rhyme for an "ane"/"ame" sound, at which point somebody suggests "My soul's waves of grain" - which is a lyric first performed in a Mystik Spyral song in the second season episode "Ill". This happens both in Daria: Camp Fear (2001) and Daria: It Happened One Nut (1999); in the latter episode, Trent vaguely remarks, "I've heard that somewhere before."
Jamie's last name White is revealed in Daria: Fair Enough (1998). The last names of Joey and Jeffy are never mentioned. Tiffany's last name Blum-Deckler is mentioned in Daria: The Invitation (1997), but it isn't a specific reference to her.