James Kirk Christopher-Fontana was the eldest child of Shaun Geoffrey Christopher and Alice Fontana, both astronauts, who lived during in the 21st century. He had one younger sibling. Furthermore, he was the grandson of John Christopher and the younger half-brother of Kevin, Katie and Rory, his father's children from his marriage to Debbie Lauderdale.
Christopher-Fontana was conceived during the USS Lewis & Clark's return voyage from Saturn to Earth and he was named after 23rd century Captain James T. Kirk, who Christopher and Fontana had met in a time travel incident. (TOS novel: The Rings of Time)