Younger siblings of autistic children are at a high likelihood (HL) of autism, language, and/or cognitive delays. Vocal complexity, a continuous measure of the developmental maturity of vocal communication, is facilitated by parent-child interaction and predicts language outcomes. This study examined whether parents' intervention fidelity to Project ImPACT, a 12-week, parent-mediated, naturalistic developmental behavioral intervention (NDBI), mediated the effect of Project ImPACT on parents' verbal responsiveness (post-intervention) and children's vocal complexity (3 months post-intervention). Participants were 54 12- to 24-month-old HL child-parent dyads who received 12 weeks of Project ImPACT (n = 28) or business-as-usual (n = 26). Project ImPACT indirectly improved both parents' verbal responsiveness and children's vocal complexity by improving parents' use of the intervention techniques. The efficacy of Project ImPACT in supporting early social communication might be attributed to how Project ImPACT helps parents improve the quality and frequency of their use of the intervention strategies across children's everyday settings.