Siblings play an important role in shaping the developmental trajectories of individuals with autism spectrum disorder. Having siblings has been associated with better social communication, non-verbal communication, and theory of mind abilities in autism spectrum disorder. However, little is known about the impact of siblings on adaptive skill growth over time, even though adaptive behavior competencies are among the strongest predictors of positive outcomes in autism spectrum disorder. This study examined the influence of sibling constellation factors, including the presence of siblings, position in birth order, gender of closest-age sibling, and gender match of sibling dyad on the adaptive behavior trajectories of individuals with autism spectrum disorder and non-spectrum disorders from ages 9 to 26 years. Participants with one or more siblings experienced faster growth rates in adaptive behavior from childhood to adulthood than participants without siblings across both Black and White participants, although effects were magnified in Black participants. Furthermore, among participants with siblings, those with male closest-age siblings and those with gender-matched siblings, irrespective of birth order, demonstrated the steepest adaptive skill growth within their respective groups over the 17-year period. Results suggest that siblings may serve an important role in improving the adaptive functioning trajectories and overall outcomes of individuals with autism spectrum disorder.