This paper explores a data base constructed from six population censuses organized in the city of Geneva between 1816 and 1843. We look at cohabitation structures in a sibling perspective. First, we show to which extent crosssectional data can inform about life course patterns. Second, we examine the transitions from one sibling status to another in the next 6 years, and the effect of several demographic, familial, and social variables on transition probabilities. Results show how the life of siblings was framed by the interactions between a (neo-)Malthusian demographic regime and a nuclear family system. Population heterogeneity resulted from the social importance of statistically marginal behaviors, as well as from the coexistence of two systems of leaving home: the socially differentiated one of the siblings who grew up in urban families, and another one of children from rural families who went through Geneva during their period of life cycle service.