Evolutionary theory and behavioral biology suggest that kinship is an organizing principle of social behavior. The neural mechanisms that mediate kinship behavior are, however, not known. Experiments confirm a sibling-approach preference in young rat pups and a sibling-avoidance-preference in older pups. Lesions of the lateral septum eliminate such kin preferences. In vivo juxta-cellular and whole-cell patch-clamp recordings in the lateral septum show multisensory neuronal responses to kin and non-kin stimuli. Non-kin odor-responsive neurons are located dorsally and kin-odor responsive neurons are located ventrally in the lateral septum. With development, the fraction of kin-responsive lateral septal neurons decrease and ongoing firing rates increase. Lesion effects, developmental changes and the ordered representation of response preferences according to kinship—an organization we refer to as nepotopy—point to a key role of the lateral septum in organizing mammalian kinship behavior.