Substance use is common among young people and can escalate into significant problems for affected individuals and their families. Family responses can influence the course of youth substance use and its consequences for family members, including parents and siblings. Family-level interventions developed to date have neglected the important role that siblings can play. This article describes a pilot test of an intervention designed to assist parents and siblings affected by youth substance use and related problems. The BEST Plus intervention consisting of professionally-led, multifamily groups sequenced over eight sessions is described with reference to the intended therapeutic processes. Professionally observed and self-reported changes for family participants including siblings suggested that the program had a beneficial therapeutic impact. This evaluation of early impacts suggests the BEST Plus program offers a promising means of assisting families to respond to substance use problems in young people.