Sibling rivalry occupies a surprisingly generative position in the depth-psychology corpus, functioning simultaneously as a clinical datum, a developmental milestone, and a mythologically resonant complex. Freud establishes the foundational terms: the arrival of a younger sibling precipitates hostility, wishes for removal, and even physical attack, phenomena he traces with unflinching directness through parental observation and clinical inference. For Freud, sibling rivalry is not incidental but constitutive — it feeds the family complex, amplifies Oedipal dynamics, and introduces the child to the grammar of rivalrous desire. Otto Rank extends this framework toward the intrauterine register, proposing a ‘brother or sister trauma’ rooted in the displaced child’s unconscious identification with the returning newcomer in the womb. Melanie Klein situates rivalry within her account of envy and jealousy, arguing that when primary envy is not excessive, sibling-directed jealousy actually serves a maturational function, distributing and thereby metabolising destructive impulses. Murray Stein offers a Jungian typological reading, showing how differences in psychological type between children set the stage for favouritism and consequent rivalry. Irvin Yalom observes the phenomenon reactivated in the group therapy situation, where it emerges as a transference pressure late in group life. Howard Sasportas, working astrologically, maps early sibling dynamics onto the third house and notes their tendency to repeat across later relational configurations. The corpus thus reveals sibling rivalry as a nodal concept at the intersection of drive theory, object relations, typology, and group dynamics.