James Kirk was the eldest son of Samuel Kirk, and brother of George Samuel Kirk, Sr. Kirk's nephew was the legendary James T. Kirk of Starfleet. (TOS novelization: Star Trek: The Motion Picture)

In Star Trek Generations, in his Nexus experience James T. Kirk finds himself at "his uncle's farm in Idaho", where he met Antonia while riding. It is unspecified whether that uncle was James Kirk, or if it was a brother of his mother.