The role expectations and behavior of siblings arise in the context of their cultural environment. While ethnology has been investigating this for years, the question of the characteristics of siblings has been neglected in historical research. In this regard, the present study on the functions of sibling relationships in early modern European courts does pioneering work. The large number of traditional letters from noble siblings of the 17th century allows a comprehensive analysis that shakes traditional notions of family as well as traditional explanatory models of early modern politics. From the historical-anthropological and micro-historical consideration of individual relationships and courses of action, the path leads again and again to the macro levels of early modern politics and courtly mentalities.