In the Pandora myth, Prometheus is Pandora’s brother-in-law through his brother Epimetheus.
Prometheus and Epimetheus are brothers in Greek mythology, and Pandora becomes Epimetheus’s wife. That makes Prometheus her brother-in-law in the ordinary family sense. The relationship is important because Prometheus is the character who understands Zeus’s hostility and warns Epimetheus against accepting gifts.
Prometheus’s own story centers on humanity. He tricks Zeus in a sacrifice and later steals fire from Olympus, giving it to humans. Zeus answers by creating Pandora as a punishment connected with that theft. The family relationship therefore connects the fire myth directly to the jar episode.
Prometheus is sometimes described as a god, but Greek sources more precisely identify him as a Titan. He belongs to the older divine generation, while Zeus belongs to the younger Olympian generation that defeats the Titans in the traditional succession story.