SummaryThis paper provides empirical evidence on sibling spillover effects in school achievement using administrative data on 230,000 siblings in England. We extend previous strategies to identify peer effects by exploiting the variation in school test scores across subjects observed at ages 11 and 16 as well as variation in peer quality between siblings. We find a statistically significant positive spillover effect from the older to the younger sibling. Sibling spillovers account for a non‐negligible proportion of the attainment gap between low‐ and higher income pupils in England.