Younger siblings of children with autism spectrum disorders (HR-Sibs) are at elevated risk for social communication deficits and language delays. One way to mitigate this risk early, before these deficits and delays become clear or impairing, may be to equip parents to use the strategies taught through the "Improving Parents as Communication Teachers" (ImPACT) intervention. In this randomized control trial, 97 HR-Sibs (mean age 14 months) and their primary caregiver were randomized to either ImPACT or a business-as-usual control group. Our "a priori," preregistered hypothesis was that parents' participation in ImPACT training would indirectly attenuate younger siblings' social communication challenges 9 months later by sequentially increasing parents' use of ImPACT strategies immediately after the intervention phase and by improving children's midpoint proximal skills (e.g. motor imitation or intentional communication). Results supported this prediction.