Family
- father Michael Tell: biological father; married to Patty Duke for less than a month; genetic testing proved that he was Sean's natural father
- father John Astin: Astin was raised to believe that John was his natural father, but genetic testing proved otherwise
- mother Patty Duke: former SAG president; was professionally known as Patty Duke Astin during marriage to John Astin from 1972 to 1985
- half-brother David Astin: older; father John Astin
- half-brother Allen J Astin: born in 1961; father, John Astin
- half-brother Thomas E Astin: born in 1965; father, John Astin
- half-brother Mackenzie Astin: born in May 1973
- half-brother Kevin Michael Pearce: younger; mother, Patty Duke
- daughter Alexandra Louise Astin: born on November 23, 1996; played Elanor, the daughter of Sam Gamgee in "Lord of the Rings" trilogy of films
- daughter Elizabeth Louise Astin: born August 6 2002; mother, Christine Astin
- daughter Isabella Louise Astin: born July 22, 2005; mother, Christine Astin
