BackgroundObjective Having a child diagnosed with cancer is stressful for the whole family and may cause significant psychological impact on parents and siblings. Chronic, life-altering diseases may also have similar effects in siblings due to the daily life changes in the family to accommodate the child with chronic disease. We investigated the impact of having a sibling with cancer or type 1 diabetes mellitus (T1DM) on the psychological features of adolescents, with particular focus on self-conscious emotions such as guilt and shame-which are associated with the development of psychopathologies. Method Sixty-four children who were siblings of patients diagnosed with cancer (CS group), 54 children who were siblings of patients diagnosed with T1DM (DMS group), and 200 adolescents with siblings who did not have any chronic disease (control group) were included in the study. The CS group was also divided into two subgroups with respect to cancer type (leukemia and non-leukemia). Feelings of guilt and shame were evaluated via the Test of Self-Consciousness Affect for Adolescents (TOSCA-A). The Children's Depression Inventory (CDI) and the State-Trait Anxiety Inventory (STAI) were used to determine the levels of depression and anxiety symptoms. Comparisons between groups were performed and within-group directional relationships between scores were analyzed. Results Guilt scores were significantly higher in CSs than controls (p = 0.009), and the guilt scores of CSs and DMSs were similar (p = 0.508). Other subdimension scores obtained from the TOSCA-A and the CDI and STAI scores were similar in all three groups. In the CS group, externalization scores of siblings with leukemia were significantly higher than that of siblings with non-leukemia cancer. Although shame scores were similar in the CS, DMS, and control groups, shame scores were found to be positively correlated with CDI and STAI total scores in each group (p < 0.05 for all), whereas guilt scores did not demonstrate any significant correlations. Conclusion Our results support prior studies in showing that CSs feel a greater level of guilt compared to adolescents without disease-stricken siblings, whereas, interestingly, CSs and DMSs were found to experience similar levels of guilt. Despite lack of significant increase in the CS and DMS groups, shame levels were positively correlated with depression and anxiety scores in all groups, but the lower correlation coefficients for the CS group indicate the presence of other factors influencing this relationship. We believe our results warrant the need for future studies evaluating the needs of the siblings of children with other chronic diseases, preferably with longitudinal follow-up to determine situations associated with need for psychosocial support.