Prometheus is Pandora’s brother-in-law in Greek mythology because he is Epimetheus’s brother.
Prometheus and Epimetheus are both Titans and are presented as brothers in the mythic tradition. Prometheus is associated with foresight, while Epimetheus is associated with afterthought. This contrast is built into their Greek names and helps explain their different responses to Zeus’s plan.
Prometheus warned Epimetheus not to accept gifts from Zeus. Epimetheus ignored the warning and accepted Pandora, who later opened the jar or container. Prometheus therefore connects the Pandora story to the earlier myth in which he steals fire for humans.
The relationship is sometimes obscured because modern retellings focus on Pandora and Prometheus as separate protagonists. Pandora is not generally described as Prometheus’s wife in Hesiod’s account; she is paired with Epimetheus. That distinction makes “brother-in-law” the precise family relationship supported by the mythic genealogy.