Background: Siblings of adults with intellectual and developmental disabilities (IDD) often support their brothers and sisters through caregiving and guardianship. Methods: In this qualitative study, the knowledge and views of 10 adult siblings were explored. Results & Conclusions: The tripartite impact of limited knowledge of guardianship and alternatives, the viewpoint of full guardianship as necessary and the desired/anticipated roles of siblings combined to create the Sibling Reciprocal Effect (SRE). The present authors define SRE as the phenomenon of siblings to recognize the applicability of complementary forms of guardianship for other adults with IDD, but fail to see the advantage of available decision-making alternatives with their own brothers/sisters. Instead, siblings defer to full guardianship as the preferred mechanism for decision making. Implications for practitioners include informing families of the full range of options for supporting persons with IDD in decision making. Future research suggestions include examining the elements of the SRE and siblings' knowledge regarding guardianship and the alternatives.