["Journal of Policy and Practice in Intellectual Disabilities, Volume 23, Issue 2, June 2026. ", "\nABSTRACT\nSiblings play a vital role in each other's lives. When one sibling has an intellectual or neurodevelopmental disability, this may influence the typically developing sibling's emotional responses and the quality of the siblings' current and future relationships. This study examined a predictive model of emotional responses, which were categorized as active negative, passive negative, and positive, among emerging adult siblings of individuals with Down syndrome or autism. The model assessed the influence of optimism, with perceived social support evaluated as a mediating variable. The data indicated that siblings who have a brother or sister with autism experienced significantly greater active and passive negative emotions than those with a brother or sister with Down syndrome, whereas the latter reported higher levels of positive emotions, optimism, and perceived social support. Gender differences emerged, with sisters of individuals with DS/autism expressing more positive emotions than brothers. Mediation analyzes revealed that perceived social support significantly mediated the relationship between optimism and both passive negative and positive emotions, but not between optimism and active negative emotions. These findings highlight the sometimes‐challenging emotional responses by typically developing siblings, particularly those of individuals with autism, and highlight the protective role of perceived social support. The study emphasizes the importance of developing targeted interventions to support sibling well‐being, especially among male siblings of individuals with autism.\n"]