Jacques Lacan’s reformulation of the Oedipus complex shaped contemporary psychoanalysis. Using schemas, he illustrated both identity formation and its underlying structure. By analyzing mother and father as functions, Lacan identified how the subject’s “I” emerges in imaginary, and symbolic dimensions. The maternal function operates through the mirror stage, while the paternal function introduces the subject into the Symbolic order. These functions are universal, since every subject must undergo their alienating effects to become, yet they are actualized throughout lived experience. Therefore, the complexes are simultaneously universal in form and contingent in expression. This universal-contingent tension provides a template for interpreting other concepts, including ideological formations. Building on it, I propose a third sibling function, derived from Lacan’s early reflections on the intrusion complex, which mediates identity development. Siblings act as counterparts for identification but also provoke rivalry and jealousy by competing for the object ‘a’ and parental gaze. This dynamic frequently pushes the subject toward the symbolic order, where identification with the Law offers the illusion of stability. By unconsciously selecting different aspects of the Other, siblings maintain familial equilibrium and secure parental attention, revealing their mediating role. Using a structural approach in the dialectic of reading, I argue that the sibling function illuminates the formation and appeal of ideologies. Its contingent dimension can be seen in diverse sibling relations and ideological positions as specific modes of identification with the Other. Simultaneously, the persistent human need for ideology reflects a universal mediating function analogous to the sibling’s role, suggesting that ideologies may also be grounded in a universal sibling-like mediating structure.