Background: Siblings of people with intellectual and developmental disabilities (IDD) often assume key roles to support their brothers and sisters. For people with more significant support needs, siblings may undertake additional roles and responsibilities throughout their lives. The purpose of the present study was to identify and describe the roles of adult siblings who have a brother or sister with severe IDD. Method: Seventy-nine adult siblings from 19 to 72 years of age completed an online survey with open-ended questions about the roles they play in their relationships with their brother or sister. Results: Thematic analysis resulted in identification of several roles including caregiver, friend (social partner), advocate, legal representative, sibling (teacher/role model), leisure planner and informal service coordinator. Conclusion: Siblings assume key roles in the lives of people with IDD and need support from family and professionals to perform these roles.