In this second of two primary papers, we examined moderators of treatment effects for younger siblings of children with autism spectrum disorders whose parents were taught to use a parent-implemented intervention called "Improving Parents As Communication Teachers". Investigators randomized 97 high-risk siblings and their primary parent to either the Improving Parents As Communication Teachers or control group, used intent-to-treat analysis, and used assessors and coders who were blinded to group assignment. We hypothesized that a cumulative risk score (incorporating younger siblings' sex, multiplex status, and behavioral risk) would moderate the effect of Improving Parents As Communication Teachers on younger siblings' proximal skills related to their continuously measured communication challenges. Pre-intervention level of parents' depressive symptoms was the proposed moderator of Improving Parents As Communication Teachers on parenting-related stress and parenting efficacy. In high-risk siblings with no additional risk factors (i.e. girls with only one older sibling with autism spectrum disorder and who score at low risk on an autism spectrum disorder screen), parental receipt of Improving Parents As Communication Teachers training had indirect effects on children's expressive language ability or autism spectrum disorder diagnosis through earlier effects on high-risk siblings' intentional communication or expressive vocabulary. "Improving Parents As Communication Teachers" intervention did not show moderated or total effects on parenting-related stress or parenting efficacy. [For the first primary paper, "Parent Utilization of ImPACT Intervention Strategies Is a Mediator of Proximal Then Distal Social Communication Outcomes in Younger Siblings of Children with ASD," see EJ1284254.]