As individuals with intellectual and developmental disabilities (IDD) live longer and begin to outlive their parents, siblings take on greater supportive roles including advocacy. Yet, little is known about the ways in which siblings advocate with and for their brothers and sisters with IDD as well as for broad, systemic changes. In this study, we conducted four focus groups (N = 18) with siblings of individuals with IDD. We found that siblings defined and engaged in case advocacy (i.e., advocacy on behalf of their brothers and sisters with IDD) and cause advocacy (i.e., advocacy for larger systemic changes). Regarding case advocacy, siblings were motivated to advocate to secure appropriate services for their brothers and sisters. For cause advocacy, siblings attempted to create sweeping changes for individuals with IDD by educating others and participating in collective advocacy methods. Regardless of the type of advocacy, all siblings felt they needed more information and peer support to effectively advocate. Implications for research, policy, and practice are discussed.