Few studies have investigated the causal spillover effects of compulsory education on children's siblings. Using a regression discontinuity method, I find that Hong Kong's 1971 free compulsory primary education policy reduced the dropout probability for the eldest siblings of full policy beneficiaries, especially for children in low-income families. Having younger brothers who were full policy beneficiaries did not affect the elder child's educational attainment. Having younger sisters who were full policy beneficiaries increased the eldest child's educational attainment, especially for the eldest sister. The results shed light on how sibling sex composition might affect intra-household resource allocation of human capital investment among children.