Research and clinical practice in first-episode psychosis (FEP) has predominantly focused on parents and partners and has neglected siblings needs. This study aims to characterize the sibling relationship in FEP and to examine the illness-related variables that influence this relationship.Survey methodology explored the experience of 157 siblings during the first 18 months of their brother or sister's treatment. The Adult Sibling Relationship Questionnaire was used to measure the warmth, conflict, and rivalry within the relationship. A series of multivariate regression analyses were conducted to determine the relationship between illness characteristics and sibling relationship.When the young person experiencing FEP had a period of untreated psychosis longer than six months, required more than one hospital admission, had persisting psychotic symptoms, continued to use substances, and/or had a history of physical violence, warmth within the sibling relationship deteriorated. Regression analysis revealed that a history of violence was a significant predictor of the warmth, conflict, and rivalry within the sibling relationship. Suicide attempts were a significant predictor of conflict.This study has established associations between the sibling relationship and illness-related variables. This study promotes consideration of the importance of including siblings in early intervention. Given the powerful role this relationship can have as a protective factor, this study could inform future interventions involving siblings.