There is increasing recognition of the potential for sibling relationships to promote the well‐being of youth in out‐of‐home care. Efficacious interventions now exist to strengthen the sibling relationships for youth in out‐of‐home care, yet the processes through which these interventions work to enhance sibling relationships remains largely speculative. The current study therefore aimed to identify the critical components of an efficacious dyadic relationship enhancement intervention for siblings in foster care through a secondary analysis of fidelity of implementation and trial outcome data. Data for 168 youth from the treatment condition of the Supporting Siblings in Foster Care study were analysed. Fidelity of implementation was assessed across seven intervention domains: Sibs 4 Life, Positive/Sib Thinking, Activity Planning, Problem Solving, Cooperation, Managing Feelings and Adult Allies. Trial outcome data were drawn from efficacy test results and included a multi‐agent construct of sibling relationship quality. Descriptive statistics detailed intervention implementation, and hierarchical linear models examined associations between intervention coverage, comprehension, and engagement, and 18‐month improvements to the sibling relationship. Results indicate high fidelity to implementation, and Positive/Sib Thinking was the primary intervention domain associated with sibling relationship improvements. Considerations for future mechanisms‐based intervention research with siblings in out‐of‐home care are provided.