Many siblings anticipate fulfilling caregiving roles for their brothers and sisters with intellectual and developmental disabilities (IDD). Given these roles and the importance of supported decision making, it is crucial to understand how individuals with IDD and their siblings make decisions. Using dyadic interviews, we examined the perspectives of nine sibling dyads (N = 18) about decision making in relation to self-determination, independent living, and employment. The ages of participants ranged from 19 to 57. Data were analyzed using constant comparative analysis to identify themes. Decision making was characterized by: parents and siblings primarily identifying courses of action; the probability of respective consequences based on the person-environment fit; and the role of the sibling in making the final decision. Characteristics related to the individual with IDD, the family, the sibling, and the environment impacted decision making. Individuals with IDD were more likely to make their own decisions about leisure activities; however, siblings were more likely to make formal decisions for their brothers and sisters.