Evidence from industrial nations suggests that sibling composition is associated with children's educational attainment, particularly if parents face resource constraints. If sibling composition is associated with educational attainment, then those associations should be stronger in poor societies of developing nations. We use data from a pre-industrial society of native Amazonians in Bolivia and found that school-age (5-16) girls or boys with an additional older sibling, particularly an older brother, were less likely to enroll in school and had fewer years of completed schooling. Because older siblings affected the school attainment of younger siblings, older siblings lowered a child's academic skills. Unlike their peers in rich households, younger sisters in poor households were less likely to enroll in school if they had an older sibling. The results lend cross-cultural support to the hypothesis that resource constraint is associated with children's human capital accumulation. (Contains 1 figure and 6 tables.)