The purpose of this study was to measure the identity development of individuals who had siblings with disabilities. We compared three sub-populations, namely, only children, individuals who had typical siblings, and individuals who had siblings with disabilities. We developed a survey that included questions on attachment, identity, socio-emotional development, family dynamics, sibling relationships, and responsibility toward siblings with disabilities. Totally, 338 participants completed the survey; out of which 145 had siblings with disabilities, 104 had typical siblings, and 89 were only children. Our results demonstrated that participants differed on several key aspects, including asking for help, expressing emotions, formation, and reliance on friendships, reliance on parental figures, receiving attention from parents, expectations from parents, and reaction to the diagnosis. Our study is one of the few studies that has explored the differences in identity development, socio-emotional development, and family dynamics in adults who are only children, individuals with typical siblings, and individuals with siblings with disabilities. Our study extends the literature in understanding sibling relationships and responsibilities toward siblings with disabilities.