What is the name of Captain Kirk's brother, found dead in 'Operation -- Annihilate!'?

Captain Kirk’s brother found dead in “Operation—Annihilate!” is George Samuel Kirk. He is usually called Sam Kirk, distinguishing him from Captain James T. Kirk.

Sam lived on the Federation colony Deneva with his wife, Aurelan, and their son, Peter. The Enterprise arrives after tracing a wave of mass insanity across several worlds. The crew discovers that parasitic organisms have taken control of many colonists, causing violent and irrational behavior.

Sam is already dead when Kirk, Spock, McCoy, and the landing party reach the Kirk family home. His death gives the episode a personal stake for the captain, while Aurelan and Peter show the continuing effects of the infestation. The parasites are eventually destroyed through intense ultraviolet radiation.

A common mix-up is confusing Sam with George Kirk, James Kirk’s father in some later Star Trek continuity, or with David Marcus, Kirk’s son. “Operation—Annihilate!” identifies the brother as Sam, whose fuller name is George Samuel Kirk.