We examined types of disabilities in siblings from a large sample of families of adolescents and adults with autism spectrum disorders (ASD) and the impact of another child with a disability on maternal and family well-being. The most frequent disabilities in siblings were attention and hyperactivity (4.6%) and autism spectrum (2.4%) disorders and psychiatric (2.1%) and learning (2.0%) disabilities. Mothers parenting another child with a disability (in addition to the child with ASD) had higher levels of depressive symptoms and anxiety and lower family adaptability and cohesion compared with mothers whose only child with a disability had ASD (matched on child age and family size). Findings are discussed with respect to understanding the needs of such families, including service provision. (Contains 5 tables.)