Beginning in the late eighteenth century, Europeans embarked on a new way of classifying the world, devising genealogies that determined degrees of relatedness by tracing heritage through common ancestry. This methodology organized historical systems into family trees, transforming the closest contemporaneous terms on trees of languages, religions, races, nations, species, or individuals into siblings. Encompassing political fraternity, sister languages, racial discourse on brotherhood, evolutionary sibling species, and intense, often incestuously inclined brother-sister bonds in literature, siblinghood stands out as a ubiquitous-yet unacknowledged-conceptual touchstone across the European long nineteenth century. In all such systems the sibling term, not-quite-same and not-quite-other, serves as an active fault line, necessary for and yet continuously destabilizing definition and classification. In her provocative book, Stefani Engelstein explores the pervasive significance of sibling structures and their essential role in the modern organization of knowledge and identity. Sibling Action argues that this relational paradigm came to structure the modern subject, life sciences, human sciences, and collective identities such as race, religion, and gender. Engelstein considers theoretical constructions of subjectivity through Sophocles' Antigone; fraternal equality and its exclusion of sisters in political rhetoric; the intertwining of economic and kinship theory by Friedrich Engels and Claude Levi-Strauss; Darwin and his contemporaries' accounts of speciation; anthropological and philological depictions of Muslims and Jews at the margins of Europe; and evolutionary psychology's theorizing around the incest taboo. Integrating close readings across the disciplines with panoramic intellectual history and arresting literary interpretations, Sibling Action presents a compelling new understanding of systems of knowledge and provides the foundation for less confrontational formulations of belonging, identity, and agency.