This paper examines causal sibling spillover effects among students from different family backgrounds in elementary and middle school. Family backgrounds are captured by race, household structure, mothers’ educational attainments, and school poverty. Exploiting discontinuities in school starting age created by North Carolina school entry laws, we adopt a quasi-experimental approach and compare test scores of public school students whose older siblings were born shortly before and after the school entry cutoff date. We find that individuals whose older siblings were born shortly after the school entry cutoff date have significantly higher scores in middle school, and that this positive spillover effect is moderated by family background. We estimate that these spillover effects account for more than one-third of observed statistical associations in test scores between siblings, and the magnitude is much larger for families in which the mother is a high-school dropout, black, single, or living in a low-income district. Our results suggest that educational spillover effects from older to younger siblings lead to greater divergence in academic outcomes between families.