We investigated sibling relationships and wellbeing in adolescents and adults with a sibling with an autism spectrum disorder (ASD). Adolescents engaged in more shared activities than did adults. Adolescents reported greater social support, greater use of emotion-focused coping strategies, and less use of problem-focused coping than adults. In adulthood, females with a sister with ASD reported the most positive affect in the sibling relationship and men with a sister with ASD the least. Adolescents engaged in more shared activities and reported more positive affect in their sibling relationship when their sibling with ASD had fewer behavior problems; greater use of problem-focused coping buffered the negative effects of behavior problems on sibling engagement. For adults, more shared activities were observed when the sibling with ASD was younger in age and had fewer behavior problems; greater positive affect in sibling relationships was predicted by greater parental support.