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Why do some kinship systems use the same term for parallel cousins and siblings?

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consanguinity (Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above

In many societies, kinship systems use the same terms for siblings and parallel cousins to reflect their shared membership within a specific family unit or lineage. This practice is common in cultures that emphasize unilineal descent, where family membership is traced exclusively through either the mother’s or the father’s line.12

By grouping parallel cousins—the children of a father’s brother or a mother’s sister—with siblings, these systems highlight that these individuals belong to the same family unit as "ego" (the individual designated as the starting point for genealogical reckoning). This classification serves to distinguish them from cross-cousins, who are considered outside of that immediate lineage. Consequently, this terminology often reinforces social norms, such as exogamy, by clearly marking cross-cousins as appropriate marriage partners while treating parallel cousins as close kin, often subject to incest taboos.12

Ultimately, these kinship terminology systems prioritize social and structural relationships over strictly biological distinctions.2