Objective This study examined social functioning among siblings of children with cancer. Method A case–control design was applied to school- and home-based data from multiple informants (peers, teachers, mothers, and self). Social reputation and peer acceptance within the classroom was compared for 87 siblings (aged 8–16 years) and 256 demographically matched peers. Self-perceptions of peer relationships and parent-reported social competence were examined among 67 siblings and 67 matched comparisons. Results Peer reports (N = 1,633) indicated no differences between siblings and comparisons for social reputation, number of friendships, reciprocated friendships, or peer acceptance. Self-reported prosocial behavior and teacher-reported likability were higher for siblings than comparisons. Self-reported loneliness, friendship quality, and perceived social support did not differ between groups. Mothers reported less involvement in activities and poorer school performance for siblings than comparisons. Conclusions Peer relationships of siblings of children with cancer are similar to classmates, though they experience small decrements in activity participation and school performance.