Abstract Objective To describe psychosocial outcomes among adult siblings of very long‐term childhood cancer survivors (CCS), to compare these outcomes to reference populations and to identify factors associated with siblings' psychosocial outcomes. Methods Siblings of survivors (diagnosed <18 years old, between 1963 and 2001, >5 years since diagnosis) of the Dutch Childhood Cancer Survivor Study DCCSS‐LATER cohort were invited to complete questionnaires on HRQoL (TNO‐AZL Questionnaire for Adult's HRQoL), anxiety/depression (Hospital Anxiety and Depression Scale), post‐traumatic stress (Self‐Rating Scale for Post‐traumatic Stress Disorder), self‐esteem (Rosenberg Self‐Esteem Scale) and benefit and burden (Benefit and Burden Scale for Children). Outcomes were compared to a reference group if available, using Mann‐Whitney U and chi‐Square tests. Associations of siblings' sociodemographic and CCS’ cancer‐related characteristics with the outcomes were assessed with mixed model analysis. Results Five hundred five siblings (response rate 34%, 64% female, mean age 37.5, mean time since diagnosis 29.5) of 412 CCS participated. Siblings had comparable HRQoL, anxiety and self‐esteem to references with no or small differences ( r = 0.08−0.15, p < 0.05) and less depression. Proportions of symptomatic PTSD were very small (0.4%−0.6%). Effect sizes of associations of siblings' sociodemographic and CCS cancer‐related characteristics were mostly small to medium ( β = 0.19−0.67, p < 0.05) and no clear trend was found in the studied associated factors for worse outcomes. Conclusions On the very long‐term, siblings do not have impaired psychosocial functioning compared to references. Cancer‐related factors seem not to impact siblings' psychosocial functioning. Early support and education remain essential to prevent long‐term consequences.