The present study examined 97 adolescent siblings of individuals with autism spectrum disorder (ASD), intellectual and developmental disabilities (IDD), or no disabilities. Siblings reported on their feelings toward their brother or sister (anxiety, hostility, and positive affect), and parents reported on general optimism, child behavior problems, and perceptions of how the child impacts the family, including the sibling. There were no differences between siblings of individuals with ASD and siblings of individuals with IDD on any sibling self-reported feelings toward their brother or sister, though parents of individuals with ASD reported significantly less optimism and more negative perception of the child's impact on the family than did parents of children with IDD or no disability.