Introduction: The presence of comorbid mental illness in individuals with intellectual and developmental disabilities (IDD) has been shown to have additional negative impact on parents and caregivers. However, the impact of such dual diagnoses on typically developing siblings has yet to be examined. Methods: Parents and typically developing adolescent siblings of a child with IDD from 49 families completed online surveys about the presence or absence of comorbid mental illness, target-child behavior problems, impact on the family, and sibling emotions toward the child with IDD. T-tests were used to compare results of families with a child with both IDD and MI and families with a child with IDD only. Results: Parents reported more behavior problems in children with comorbid IDD and MI, and a greater overall impact on the family. Adolescent siblings in the comorbid IDD/MI group reported higher levels of hostility, anxiety, and dysphoria toward their sibling, though there were no group differences in positive affect. Hierarchical linear regression results found that group membership was uniquely related to sibling dysphoria beyond target-child behavior problems. Conclusions: Having a brother or sister with comorbid mental illness and IDD seems to contribute to more negative feelings from adolescent siblings, though the absence of MI does not relate to more positive feelings. Results suggest that families of children with IDD/MI may need to be more proactive in fostering healthy sibling relationships.