OBJECTIVES: To investigate the evidence concerning the psychosocial functions among young adults (18-29 years) with a sibling diagnosed with cancer and the psychosocial interventions developed for this group. METHODS: Utilising the Joanna Briggs Institute's guidelines for scoping reviews, a comprehensive search across five databases, including PubMed, Scopus, CINAHL, PsycInfo, and the Cochrane Library, was conducted. A chain search and a search for grey literature in Google Scholar complemented the search. RESULTS: A total of 1937 titles and abstracts were screened, with 250 full-text articles read for inclusion. Seven studies met the inclusion criteria consisting of qualitative (n = 2), quantitative (n = 3), and mixed method (n = 2). Two of these described interventions; a family-centred intervention and a person-centred intervention specifically targeted at siblings. Through data analysis of the results from the seven studies, three themes emerged: (1) the sibling's inner struggle: coping strategies, emotional suppression, and ambivalent feelings; (2) informed understanding: the vital role of communication; and (3) seeking support: addressing social estrangement and strengthening familial bonds. CONCLUSION: This scoping review highlights the profound emotional and psychosocial difficulties faced by young adult siblings when a sibling's cancer affects family dynamics. It summarizes findings from studies employing qualitative, quantitative, and mixed-methods methodologies. The interventions used were successful; however, the limited number of these interventions emphasizes the need for further research. This research is essential to develop strong support networks and communication strategies to help young adults aged 18 to 29 manage their complex emotional responses and family roles effectively. IMPLICATION FOR NURSING PRACTICE: To support young adults with a sibling diagnosed with cancer, a family-centered approach is essential for assessing their individual support needs. Developing interventions targeting young adult siblings and enhancing family communication may benefit both the siblings and the family as a whole.