A considerable body of research has studied the effects of siblings on child mortality through birth intervals. This research has commonly focused on older siblings. We argue that birth intervals with younger siblings may have equal or stronger effects on child mortality, even during a mother's pregnancy. Moreover, we contend that birth interval effects need to be considered only when siblings are coresident. Using longitudinal data from 29 Health and Demographic Surveillance Systems across sub-Saharan Africa, covering more than 560,000 children, we examine the proximate role of siblings and mothers in child mortality. We find that a birth interval of 24 months or more is advantageous for both older and younger siblings. The effect of a younger sibling on child mortality is more pronounced than that of an older sibling and adds to the effect of an older sibling. Moreover, child mortality is particularly low during a mother's subsequent pregnancy, contrasting the shock resulting from a younger sibling's birth. Further, we find that a mother's or sibling's absence from the household results in a higher risk of mortality, and the death of either reduces child survival up to six months before the death.