The purpose of this qualitative study was to examine perceptions of adult sibling relationships with a brother or sister with severe disabilities and the contexts affecting the relationships. Adult siblings without disabilities (N = 79) from 19 to 72 years of age completed an online survey with four open-ended questions about their relationship with their brother or sister with intellectual and developmental disabilities (IDD) and extensive or pervasive support needs. Inductive analysis yielded findings related to perceptions of the sibling relationship and contexts that influence the sibling relationship. More than half of the relationships were described as being close. The emotional impact of the relationships included feelings of guilt and joy, as well as frustration and stress that were often related to current caregiving and future planning responsibilities. The contextual factors influencing relationship development included several characteristics of their brother or sister with IDD related to his or her disability, as well as sibling proximity. Implications for research and practice related to sibling relationships are provided for professionals, families, and the siblings themselves.