The Ankave-Anga consider that brothers and sisters share the same blood but that only women, therefore sisters, transmit it to their children. The brother-sister relationship is emphasized during the male ritual cycle, which transforms a young boy into an adult man, a warrior and a father. The paper argues that this emphasis is related to the importance this culture gives to the position of the mother’s brother and to the avuncular relationship. Among all male kinship statuses, only that of maternal uncle is endowed with the capacity to influence another person’s own life and life-giving abilities. The analysis relies on diverse ethnographic materials, such as the actions a sister must perform and the taboos she must respect for her brother during the various stages of his initiation ; the content of the relationship between a married sister and her brother, and the exchanges that take place during her married life ; and the content of the relationship between a maternal uncle and his nephews and nieces.