Sibling triangles exist independent of parent-child triangles and undergo parallel development into constellations bearing significant formal and dynamic similarities to the standard parent-child oedipal relationships. They may exert definitive effects on the individual's identifications, adult object choices, and patterns of relating. Recognition of such constellations and their outcomes is often crucial to successful therapy. A developmental line is delineated with emphasis on the recapitulation throughout development of oedipal sibling issues. Speculations are offered about the possible factors responsible for pathological outcomes of oedipal sibling triangles, and about why, in many cases, oedipal sibling experience may be more influential in development than oedipal parental experience.