Background Forcibly displaced adolescents face increased risks for mental illness and distress, with adolescent girls disproportionately affected in part due to heightened gender inequity. Although the family unit has the potential to promote healthy development in adolescents, few family interventions have employed a gender transformative approach or included male siblings to maximize benefits for adolescent girls. Methods This study will assess a whole-family and gender transformative intervention-Sibling Support for Adolescent Girls in Emergencies (SSAGE)-to prevent mental health disorders among adolescent girls in Colombia who were recently and forcibly displaced from Venezuela. The study will employ a hybrid type 1 effectiveness-implementation pilot randomized control trial (RCT) to test the program’s effectiveness to explore determinants of implementation to establish the feasibility, acceptability, and fidelity of SSAGE. To address these aims, we will enroll 180 recently arrived, forcibly displaced adolescent girls in an RCT and examine the program’s effectiveness in the prevention of mental illness (through reduction in anxiety, depression, interpersonal sensitivity, and somatization symptoms) one-month post-intervention. We will use contextually adapted to collect data on the hypothesized mechanistic pathways, including family attachment, gender-equitable family functioning, self-esteem, and coping strategies. The implementation evaluation will employ mixed methods to assess the program’s feasibility, acceptability, fidelity, and barriers and facilitators to successful implementation. Discussion Findings can support humanitarian program implementation, as well as inform policy to support adolescent girls’ mental health and to prevent the myriad disorders that can arise as a result of exposure to displacement, conflict, and inequitable gender norms.