Taking a family systems perspective, several research studies have shown that the family context (especially maternal well-being) predicts psychological adjustment in children with autism. This work has mainly focused on dyadic relationships in the family (especially parent-child reciprocal effects). In the present study, we extended a systems perspective in autism family research to a triad involving the child with autism, their mother, and a sibling, and also adopted a longitudinal design. Mothers from 60 families of children with autism reported on their own depression, and the behavior problems and pro-social behavior of their child with autism and a sibling. Results from longitudinal regression models suggested that earlier levels of maternal depression and sibling pro-social behavior did not have an independent effect on the behavior problems or pro-social behavior of children with autism 2.5-3 years later. Earlier levels of sibling behavior problems were associated with increased behavior problems of the child with autism 2.5-3 years later. Although replication is required, these are the first data to suggest that outcomes for children with autism may be affected by their siblings’ psychological adjustment. The methodology of longitudinal family systems analysis of triadic relationships has important research and practical implications.