The purpose of the study was to investigate the effects of play materials on the interactions between a child with autism and her sibling. Three conditions were assessed: open choice, materials chosen by the child with autism, and materials chosen by the typically developing sibling. Within each activity, measures of social interactions were assessed. Results of the assessment showed that more interactions occurred with a material chosen by the child with autism. After sibling training (targeting specific teaching skills), social interactions remained highest in the condition with materials chosen by the child with autism. The results are discussed in terms of a material assessment to optimize sibling training conditions and the importance of sibling relationships.