This article investigates the household determinants of teen marriage in Ethiopia, India, Peru, and Vietnam using data from the international Young Lives study tracking a cohort of children from the ages of 8–19 over a 15-year period. First, we offer a descriptive and comparative overview of the prevalence of teen marriage among girls in geographically selected areas of the four countries, together with their sociodemographic determinants. Second, we place a specific focus on the role of gender and sibling sex-composition in shaping the probability of getting married by age 19. Drawing on the significant cross-country heterogeneity in household context, direction of marriage payments, and prevalence of arranged marriage, we test hypotheses relating to the availability of economic resources within the household and cultural norms surrounding the order and timing of marriage. We show that in Ethiopia, India, and Vietnam, presence and number of older sisters in the household are associated with a 30–50 percent lower likelihood of teen marriage, while the association is null in Peru. Also, we show that having a girl as next-youngest sibling does not significantly affect girls’ likelihood of experiencing teen marriage, except in Ethiopia. Our results combined support theories of family-level resource constraints over sibling rivalry hypotheses. Our findings enrich and complement existing evidence on the role of sibling sex-composition on adolescent outcomes in low- and middle-income countries.