This study examined the longitudinal associations of sibling intimacy and conflict with civic attitudes and behaviours among Chinese young adults. At two time points separated by about 12 months, questionnaire data were collected from 272 Chinese college students (mean age at Time 1 = 19.68 years; 69% female), students who studied in Hong Kong and had at least one sibling. Students rated their intimacy and conflict with their siblings and their parents at Time 1, and their civic attitudes and behaviours at both time points. Hierarchical regression revealed that, controlling for demographic characteristics and parent–child intimacy and conflict, sibling intimacy predicted increases in both civic attitudes and behaviours. Sibling conflict was a non‐significant predictor, however. Findings highlighted the roles of siblings in understanding civic development in young adulthood and the utility of targeting sibling intimacy as means to foster young adults’ positive attitudes to and active participation in civic activities.