Evidence drawn from nineteenth-century Belgian population registers shows that the presence of similarly aged siblings competing for resources within a household increases the probability of death for children younger than five, even when controlling for the preceding birth interval and multiple births. Furthermore, in this period of Belgian history, such mortality tended to cluster in certain families. The findings suggest the importance of segmenting the mortality of siblings younger than five by age group, of considering the presence of siblings as a time-varying covariate, and of factoring mortality clustering into analyses.