Background: Good sibling relationships in adulthood are known to be a protective factor for mental health. The present study examined and compared the relationships of siblings with either a healthy brother or sister or one experiencing schizophrenia. Methods: In the first phase, we ran a statistical comparison of the two sibling groups on the quality of their sibling relationships (warmth, conflict, and rivalry), emotional distress, and self-esteem. In the second phase, we looked at whether the quality of the sibling relationship modifies the impact of having a brother or sister with schizophrenia on emotional distress and self-esteem. Results: Results showed that sibling relationships in schizophrenia are less warm and are characterized by heightened rivalry and conflict. In addition, analysis revealed a mediating effect of sibling relationship on the emotional distress of siblings with a brother/sister diagnosed with schizophrenia. Conclusion: More needs to be done to enhance the mental health of adults who have a brother or sister with schizophrenia, notably via their sibling relationships.