Current literature on family social support of children and youth in out-of-home care usually focuses on contact with parents, especially mothers. However, the few studies that examine the role of siblings in the lives of youth in care show that closeness to siblings is associated with increased well-being. This study examined the self-reports of youth in Israeli residential care settings designed for youth from underprivileged backgrounds on the extent of perceived availability of support from their siblings among other sources of support, and the contribution of sibling support to various positive and negative measures of well-being and functioning. The study was based on a cluster random sample of 864 youth, aged 13–20, who completed a structured questionnaire in their residential care settings. The findings showed that siblings were highly involved in the lives of adolescents in residential care and that they were the third most common source of perceived support among the youth. The findings also showed that greater sibling support was linked with lower levels of emotional symptoms, hyperactive behaviour, conduct problems, peer problems, and overall adjustment difficulties, and with increased levels of prosocial behaviour and life satisfaction. The findings enhance our understanding of the importance of familial figures other than parents to the well-being of children and youth in residential care. The findings also have implications for intervention programmes aimed at strengthening the social support systems of children and youth in out-of-home settings in order to enhance their well-being.