This position encompasses the idea that the family concept is represented by a vertical, complementary relationship between child and parents. For decades this position was hardly disputed. The sibling relationship was set apart, or more exactly fixed on a negative, psychopathological potential, in great contrast to daily life-experience and to the cultural value-system. This view point (and its persistant dominance) is illuminated against the background of multiple motivated, collective counterprocesses and deficient, intersubjective development theories. A new and clear change that has been taken place in the concept of the sibling relationship during the last decade is expounded and discussed here. Important conclusions for the development theory are submitted. The sibling relationship is, like the parent-child relationship, an elementary experience and a transferable behaviour-model. Significant psychodynamic processes in sibling relationships are hereby described and possible variant family-dynamics resulting from the interplay between the horizontal and vertical levels are outlined. The creation of sibling relationships during children's games, of imaginary siblings and fantasies of wishing for a sibling are shown. The relevance of the sibling-dynamics for development processes and for family-dynamic processes thus becomes apparent. Important implications for the clinical-psychotherapeutic domain reinforce the demand for including the consideration of siblings in the psychodynamic discussion of the family and in the clinical setting.