Very little is known about sibling influences on child adjustment in non-Western communities. Therefore, this multi-informant study examined the longitudinal associations of sibling warmth and conflict with peer and academic adjustment and tested birth order and gender as moderators among Chinese families from Hong Kong, China. On two occasions separated by about 12 months, data were collected from two siblings in each of 189 families. Data were also collected from the mothers and class teachers of these siblings. At Time 1, older and younger siblings' ages averaged 10.06 years (SD = 1.07) and 7.82 years (SD = 0.95), respectively. Among older siblings, 31% were boys, and among younger siblings, 48% were boys. At Time 1, siblings rated their warmth and conflict with each other. At Times 1 and 2, class teachers rated siblings on their peer exclusion, prosocial/communication skills, and academic performance. At Time 1, mothers rated their warmth and conflict with each sibling and provided family demographic information. Multilevel models revealed that, controlling for mother warmth and conflict and demographic factors, sibling warmth predicted increases in prosocial/communication skills and increases in academic performance, and sibling conflict predicted decreases in academic performance. Moreover, for younger boys, sibling conflict predicted increases in peer exclusion. Theoretically, findings highlighted the unique roles of sibling warmth and conflict, as related but distinct factors, in understanding the peer and academic adjustment of Chinese children. Practically, findings pointed to the utility of improving sibling relationships to promote positive child development. (PsycInfo Database Record (c) 2021 APA, all rights reserved).