In humans, alloparents are usually thought to be grandmothers and adolescent girls. Although many studies have examined the influence of grandmothers on child outcomes, fewer have explored the effect of adolescents on such outcomes. We tested the hypothesis that in a community of Ecuadorian Shuar horticulturists, adolescent girls would have a positive influence on the growth and development of younger siblings. We measured height, weight, and skin fold thicknesses of children and young adults and computed body mass indexes (BMIs). Contrary to predictions, adolescent girls had a strong, significant negative impact and boys had a positive impact on child growth and nutrition: the age-standardized BMI of children with all adolescent sisters was 1.7 SD below the age-standardized BMI of children with all adolescent brothers. In this population, adolescent girls have many mating opportunities, whereas adolescent boys do not. It is therefore possible that instead of pursuing marriage opportunities, adolescent boys might have invested in younger siblings.