This dissertation explores sibling relationships, and relating in adult peer groups. It hypothesises that sibling relationships are important and influential to peer relationships. The sibling bond is usually the child’s first experience of a peer group. A review of the sibling literature indicates that siblings provide important experiences for each other such as recognition and approval, conflict and competition, cooperation, solidarity, and loyalty. Siblings may be crucial to the formation of identity. Whereas the relationship of group members to the group leader has been extensively researched, the sibling qualities of member-to-member interactions in groups have been relatively neglected. In attempting to ‘fill the gap’, this dissertation makes a significant contribution to our understanding of psychosocial behaviour. Participants in this study were members of mental health trainee supervision groups. I interviewed individual members about their sibling and peer relationships, and recorded group supervision meetings. These data were transcribed. Using a grounded theory research approach, I observed strong similarities between both past and present sibling relating, and current relating in adult peer groups. I developed a set of hypotheses to explain these observed similarities. The dissertation suggests that sibling relating provides a framework for our adult peer relating. The theoretical concepts of tele and transference are used to explain how patterns of relating with original siblings influence patterns of relating in adult peer groups. I hypothesise that through tele and transference, group members relate to their peers as “as-if” siblings. Tele and transference are hypothesised to structure relationships in terms of concordance and complementarity. Findings indicate that the interaction of tele and transference, and the concordance and complementarity which they help to organise, have value as an interpretive framework in furthering our understanding of the developmental movement from the original family of origin to our relating in adult peer groups.