Purpose: This study examines the effect of sibling size on children's educational attainment in Indonesia. Design/Approach/Methods: To establish causality, it exploits the exogenous variation of sibling size caused by twin birth in families. Findings: Results of instrumental variable (IV) estimation support a traditional wisdom of quantity-quality trade-off, where sibling size is negatively correlated with the completed years of schooling, educational levels, and likelihood of school attendance. Building on this, the subsample analysis reveals that the negative effect is larger for Muslim children, children with less educated mothers, and children belonging to earlier birth cohorts. Originality/Value: These findings provide insights into both population and education policymaking in developing countries.