We examined sibling relationships for children and adolescents with intellectual disability and assessed implications for their social functioning. Targets (total N = 212) had either intellectual disability, a chronic illness/physical disability, or no disability. Nontarget siblings reported on relationship quality, sibling interactions were observed, and teachers reported on social adjustment. Group comparisons highlighted the asymmetrical hierarchy and low conflict unique to siblings and targets with intellectual disability. Sibling relationships characterized by high warmth/closeness, positive affect, and few negative behaviors were predictive of fewer behavior problems for the targets at school. Both high warmth/closeness and high conflict predicted greater social competence for the targets with intellectual disability, though warmth, conflict, and sibling management had different implications depending on the sibling's gender.