In the three sections of Sibling Action, Stefani Engelstein analyzes the essential function of the sibling term in structuring the modern subject (one chapter), the modern state (two chapters), and the methodology of the life- and human-sciences (two chapters). The first section of the book explores critical theory to argue that a sibling logic disrupts stark boundaries of subjectivity and of gender through the recognition of differential degrees of otherness. In the second section of the book, Stefani Engelstein focused on the fraternal element of political and economic theories, beginning with the contract theory of Locke and Rousseau before the French Revolution, including the fraternal civic theory of Moses Mendelssohn, and continuing through Marx and Engels and up to Lévi-Strauss’s economic theory of kinship. Moving from nation to related collective identities, part III illuminates the role of the sibling term in conceptual and figural family trees built to designate the genealogical development of languages, of species, of religions, and of races. Setting boundaries around terms was also an act of paring away siblings. The recognition of such definitions as contingent rather than natural led to epistemological anxiety. An epilogue traces the descendants of the genealogical fields into their present disciplinary settings.