Using data from the “Young Lives” study of childhood poverty tracking a cohort of children from the ages of 8 to 19, this paper aims to investigate the household determinants of teen marriage and teen pregnancy in four low- and middle-income countries (LMICs), namely Ethiopia, India, Peru and Vietnam. Its contribution is twofold. First, we offer a descriptive and comparative overview of the prevalence of teen marriage and childbearing in geographically selected areas across the four countries of interest, together with their socio-demographic determinants. Second, we place a specific focus on the role of gender and sibling sex-composition in shaping the probability of getting married and/or having a child by age 19. We test the hypothesis that in contexts where resources are scarce and customs are rooted, parents tend to arrange their daughters’ marriages in order, hence girls with older sisters face a lower risk of marrying early or giving birth, all else equal. We show that, while in most countries the presence and number of older sisters in the household is associated with a 10-to-30- percent lower likelihood of teen marriage and pregnancy, the evidence weakens – and somewhat reverses in Ethiopia – once a presumably causal effect is estimated. As such, our findings enrich and complement existing evidence on the role of sibling sex-composition on later-life outcomes in LMICs.