This article reads the medieval Welsh prose tale Branwen ferch Lŷr as a narrative of kinship relations gone wrong. Using as a critical tool Frederic Jameson’s notion of the text as a space where social contradictions can be explored and resolved on a fictive level, this reading stresses how the international politics of dynastic alliance highlight the tensions already present in families in which the kinship status of half-siblings is ambiguous and inheritance practices in flux. In contrast to the traditional “peaceweaver” model, which posits marriage alliances as a method of solving existing feuds between two peoples, this reading argues that the exogamous alliance in Branwen serves to bring out conflicts previously existing within the circle of the original family. These conflicts stem from the medieval Welsh ideology of kinship operative in Branwen, which, while purportedly protective, ultimately destroys the British royal family and their hold on power.