A cousin was a familial relation.
Individuals who were cousins of each other included Troilus and Aeneas, (TV: The Myth Makers) Tegan Jovanka and Colin Frazer, (TV: Arc of Infinity) Ranulf Fitzwilliam and Geoffrey de Lacy, (TV: The King's Demons) Martha Jones and Adeola Oshodi (TV: Smith and Jones) and Willa Twiston and Becka Savage. (TV: The Witchfinders) Yasmin Khan said that her cousin had seen the Arctic Monkeys' first ever gig. (TV: It Takes You Away) Matteusz Andrzejewski slept in his cousin's car after moving out of Charlie Smith's house. (TV: The Lost)
According to one account, cousinhood was the only familial relation on Gallifrey, as Gallifreyans used Looms to reproduce after the Pythia cursed their world with sterility. The Doctor was one of forty-five cousins of the House of Lungbarrow. (PROSE: Cat's Cradle: Time's Crucible, Lungbarrow) The term was obsolete by the time of the War in Heaven, but Faction Paradox continued to use it for their most populous rank. (PROSE: The Book of the War)
The use of the word was also used to describe individuals of similar origin. The Third Doctor described the people of Atlantis and Athens as cousins. (TV: The Time Monster) A member of the Slitheen family referred to their cousins from the Blathereen and Hazrateen family. (AUDIO: The Taste of Death) The Eleventh Doctor described the Minotaur as a distant cousin of the Nimon. (TV: The God Complex) Prince Javill described his world of Espero as "the poor cousin" of Earth's other colonies. (PROSE: Halflife)