This paper estimates family spillovers in high school major choice in Sweden, where admission to oversubscribed majors is determined based on GPA. Using a regression discontinuity design, we find large sibling and intergenerational spillovers that depend on the sex mix of a dyad. Same-sex siblings copy one another, while younger brothers recoil from an older sister's choices. Fathers and mothers influence sons but not their daughters, except when a mother majors in the male-dominated program of engineering. Back-of-the-envelope calculations reveal that these within-family spillovers have sizable implications for the sex composition of majors.