Although siblings are ubiquitous in our lives and the lives of our patients, there has been no theory of sibling impact on the development of the mind and character. In an effort to encourage the development of such theory, this paper will review the major ideas that have been proposed thus far about the role of siblings on the development of the mind, including impacts of ordinal position, identity formation, sibling loss, sense of embodied self, and capacity for intimacy. The role of resistance in exploring sibling impact will be suggested, and how this may create a blind spot for analysts identifying and interpreting sibling transference and countertransference. Some ideas and case material will be presented in an effort to initiate such a theory-building process.