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In presenting two sibling pairs observed as infants and toddlers during the Margaret Mahler separation-individuation research and followed for 60 years, this paper demonstrates how a sibling relationship contributes to the development of the inner world and sense of self, and it argues that these processes need to be more fully recognized in psychoanalytic theory and practice. With a focus on how the sibling relationship affects the development of self- and object mental representations, the lateral developmental trajectory of each sibling relationship is traced in the context of the developing vertical relationship to the parents. Analyses are drawn from detailed observations during the first three years of the mother-child and sibling interaction patterns and from follow-up interviews conducted periodically from latency through late middle age, when outside measures including the Adult Attachment Interview, Object Relations Inventory, the Structured Interview of Personality Organization, and projective tests also were administered. Although both sibling pairs experienced early adversity, one pair’s AT classification was discordant, the other concordant. The concordant sibling relationship fostered separation-individuation and attachment, helping to move the attachment to the parents (and each other) to a higher level of differentiation and relatedness in later life. In the nonconcordant sibling pair, attachment to the parents was moved to a lower level of differentiation and relatedness later in life and to a lower level of differentiation in relationship to the sibling. The developmental trajectories of each sibling pair are considered in light of these classifications. The hypothesis-generating findings result in the articulation of specific areas for further research with greater numbers of subjects.