In Stefani Engelstein’s new book, “Sibling Action: The Genealogical Structure of Modernity,” the author argues that a genealogical way of thinking about global relations emerged in the nineteenth century. This way of thinking led to several notable successes, such as evolutionary theory and the outlines of a number of language families, but it simultaneously served as the foundation for massive failures that have cast long shadows – such as race theory. Engelstein focuses on the sibling link within genealogies as a way to understand their inherent instability and provide a clue to alternative, more nuanced ways of thinking about classifications.