Siblings of children with neurodevelopmental disorders (ND) are at increased risk of mental health problems. The burden on families of children with ND is exacerbated in low-income countries with limited health services and dependency on informal care systems. Yet, there is little research on family impacts of ND in non-Western settings, and no evidence-based interventions for siblings. We examined initial outcomes and feasibility of a manual-based intervention for siblings and parents of children with ND, called SIBS, delivered in Cambodia. SIBS has promising evidence from an open trial in Norway. We delivered eight groups for 52 siblings (M age = 12.7 years, SD = 2.7; 44.0% female) and 56 caregivers (M age = 43.5 years, SD = 8.5; 61.1% mothers) of 54 children with ND at the only public child mental health clinic in Cambodia. We aimed to improve sibling and parent mental health and family communication. The SIBS intervention comprises three separate sibling/parent group sessions and two joint sibling-parent dialogue sessions. Parent-reported mental health scores for siblings were higher than sibling self-report. Parent mental health problems at baseline were high, with no difference between mothers and fathers. There was significant improvement in parent mental health and parent-rated mental health for siblings from baseline to 4-month post-intervention (effect sizes d = 0.44 to 0.52). There was no change in sibling-reported mental health or family communication. Sibling- and parent-rated user satisfaction was high. We conclude that the SIBS intervention showed promise in Cambodia. However, revision of the communication component is needed.