Using hand-collected data on siblings of chairpersons in Chinese family firms, we examine the impact of the chairperson having siblings on the corporate social responsibility (CSR) of their firm. The findings suggest that when a firm has a siblings-chairperson, the firm has a better CSR rating than firms with a chairperson without siblings. Specifically, a firm with a siblings- chairperson, on average, has a CSR rating approximately 7.96% higher than a median firm’s rating. The conclusions are robust to a battery of robustness checks including a regression discontinuity research design, alternative measures of CSR, a propensity score matching sample, placebo tests, and different estimation methods. Additional analysis suggests that the mechanisms behind siblings and CSR are consistent with both competition and altruistic effects among siblings. Further analysis suggests that the positive impact of a siblings-chairperson on the CSR rating of firms is more salient when the local familism culture is stronger, when government official career advancement incentives are lower, or when the siblings are directors or CEOs of other firms. Finally, firms with a siblings-chairperson are also pro-shareholder because they consume less perquisites than firms without a siblings-chairperson. Collectively, the findings are consistent with the notion that, by having at least one sibling, a chairperson is more competitive and altruistic than a chairperson without siblings, and such behavior enhances CSR. Family structure matters in corporate practices.