When capital and labor markets are imperfect, choice sets narrow, and parents must choose how to ration available funds and time between their children. One consequence is that children become rivals for household resources. In economies with pro-male bias, such rivalries can yield gains to having relatively more sisters than brothers. Using a rich household survey from Ghana, we find that on average if children had all sisters (and no brothers) they would do roughly 25-40% better on measured health indicators than if they had all brothers (and no sisters). The effects are as large as typical quantity-quality trade-offs, and they do not differ significantly by gender.