Autism is a complex spectrum of disorders with no clear etiology or risk model. Research on early signs of autistic spectrum disorders (ASD) has depended upon retrospective data. Recent assumptions regarding increased risk led to following the development of infant and toddler siblings of children diagnosed with ASD. At this time, information about the first months of life are documented in case studies. The case presented here follows the infant-parent psychotherapy in a DIR model with a newborn sibling of a child with high functioning autism and a child with a sensory/ regulatory disorder. The author examines the transaction between the constitutional factors in the infant and the psychological components influencing development and family life. The role of the infant mental health practitioner and hypotheses regarding indicators for future research are addressed.