Researchers who study the effects of chronic illness on well siblings have generally focused on individual characteristics and their relationships with psychological adjustment. More recently, researchers suggest that sibling adjustment can be best understood within the context of the family. The purpose of this study was to examine variations in sibling behavioral adjustment in relation to mothers' perceptions of the illness experience and family life. Based on mothers' ratings on the behavior problem scale of the Child Behavior Checklist (CBCL), five siblings considered poorly adjusted and five very well-adjusted siblings were compared with respect to mothers' reports of individual family member's response to illness, illness management, parenting philosophy, presence of other stressors, availability of social supports, and impact of illness on family members and family life. Two major differences were found between mothers who rated healthy siblings either poorly or very well adjusted: (a) effects of illness on the healthy sibling, the ill child, and the marital relationship and (b) perceived controllability of the chronic illness. Devising ways of helping mothers feel confident in managing their child's illness is integral to creating an environment that promotes optimal development of their ill child and the child's siblings.