Researchers have begun to explore the role that family members play in maintaining or recovering from an eating disorder. However, little research has addressed the sibling relationship, including nurturance, quarreling, and favoritism. Self-report questionnaires were collected from 120 girls and 87 boys in middle school (N=161) or high school (N=46). Linear regressions, ANOVAs and mediation analyses were conducted to determine the effects of siblings on adolescents’ disordered eating. Sibling favoritism and modeling of bulimic behavior were the only variables significantly related to relationship quality. Our results indicate that family dynamics and structure may play a larger role in adolescent maladaptive behavior than sibling relationship quality. Additionally, sibling relationship quality, bulimic modeling and sibling favoritism may be part of a very complex process leading to disordered eating behavior. Future research should continue to utilize the sibling subsystem as a means of understanding the development of disordered eating behavior among adolescents.