This study investigated associations between sibling relationships and adjustment among 179 African American adolescent siblings (controlling for family factors) and tested moderating effects of familism values and birth order. Two-level random intercept models revealed that familism values moderated sibling relationship-adjustment linkages, suggesting that youth who reported both strong familism values and harmonious sibling relationships showed the most positive outcomes. These effects were more consistent for older than for younger siblings. Findings highlight the role of cultural values and birth order in shaping sibling influence processes.