Studies are needed to better understand the broad autism phenotype in young siblings of children with autism. Cognitive, adaptive, social, imitation, play, and language abilities were examined in 42 non-autistic siblings and 20 toddlers with no family history of autism, ages 18-27 months. Siblings, as a group, were below average in expressive language and composite IQ, had lower mean receptive language, adaptive behavior, and social communication skills, and used fewer words, distal gestures, and responsive social smiles than comparison children. Additionally, parents reported social impairments in siblings by 13 months of age. These results suggest that the development of young non-autistic siblings is affected at an early age and, thus, should be closely monitored, with appropriate interventions implemented as needed.