We investigate the effect of having opposite sex siblings on cognitive and noncognitive skills of children in the United States at the onset of formal education. Our identification strategy rests on the assumption that, conditional on covariates, the sibling sex composition of the two firstborn children in a family is arguably exogenous. With regard to cognitive skills, learning skills and self-control measured in kindergarten, we find that boys benefit from having a sister, while there is no effect for girls. We also find evidence for the effect fading out as early as first grade.