This paper examines the impact of sibling gender on adolescent experiences and adult labor market outcomes for a recent cohort of U.S. women. We document an earnings penalty from the presence of a younger brother (relative to a younger sister), finding that a next-youngest brother reduces adult earnings by about 7%. Using rich data on parent-child interactions, parents’ expectations, disruptive behaviors, and adult outcomes, we provide a first step at examining the mechanisms behind this result. We find that brothers reduce parents’ expectations and school monitoring of female children while also increasing females’ propensity to engage in more traditionally feminine tasks. These factors help explain a portion of the labor market penalty from brothers.