Sibling aggression among maltreated children placed in foster homes is linked to other externalizing problems and placement disruption. The reduction of sibling conflict and aggression may be achieved via a multicomponent ecologically focused intervention for families in the foster care system. The focus of the study is to evaluate the feasibility and short-term effectiveness of a transtheoretical intervention model targeting sibling pairs and their foster parent that integrates family systems, social learning theory, and a conflict mediation perspective. In this pilot study, sibling pairs (N=22) and their foster parent were randomized into a three-component intervention (n=13) or a comparison (n=9) group. Promoting Sibling Bonds (PSB) is an 8-week prevention intervention targeting maltreated sibling pairs ages 5–11 years placed together in a foster home. The Siblings, Parent, and Joint components were delivered in a program package at the foster agency by a trained two-clinician team. Average attendance across program components was 73%. Outcomes in four areas were gathered at pre-and post-intervention: observed sibling interaction quality (positive and negative) including conflict during play, and foster parent reports of mediation strategies and sibling aggression in the foster home. At post-intervention, adjusting for baseline scores and child age, intervention pairs showed higher positive (p<.001) and negative (p<.05) interaction quality, and lower sibling conflict during play (p <.01) than comparison pairs. Foster parents in the intervention group reported a higher number of conflict mediation strategies than those in the comparison group (p <.001). Foster parents in the intervention group reported lower sibling physical aggression from the older toward the younger child than those in the comparison group (p <.05). Data suggest that the PSB intervention is a promising approach to reduce conflict and promote parental mediation which together may reduce sibling aggression in the foster home.