The kin relationship, rather than a biological one, dictates the incest taboo (Tonkinson 1991). In traditional societies, the incest taboo extends to all the members of one’s own moiety, with certain exceptions during sacred rituals. For example, during the defloration ceremony, a man inserts the defloration boomerang into a woman whose formal relationship to him is roughly the equivalent of his wife’s mother; he then has coitus with her as a sacred ritual considered important from the point of view of fertility.
все там же
Современные нравы для сравнения