The predictions of resource dilution and sibship gender composition models of educational investment are tested using the Japanese Nationwide Survey on Families (N = 6,985). Japan is an important case because of its postindustrial economy, coupled with high levels of dependence on parental investment to attend a university and persisting gender inequality in educational attainment. In previous between-family analyses of educational attainment in Japan, boys were found to drain resources from their sisters. The within-family, multilevel models of parental educational investments in this analysis show that girls with college-educated brothers fare better than their peers without brothers. An alternative model incorporating the educational investments received by brothers in the same family is proposed.