Although they will often serve as caregivers for their brothers-sisters with intellectual and developmental disabilities (IDD), adult siblings are rarely included in future planning. This study examined 495 American siblings who completed a web-based questionnaire about themselves, their brother-sister with IDD, parents and whether their families completed 11 future planning activities. Although virtually all families completed some future planning, on average, families completed slightly over half of the 11 activities (75% completed eight or fewer). Families more frequently identified a successor to current caregivers and engaged in planning discussions with one another and with the brother-sister; least often, families completed a letter of intent or began securing residential placements. Future planning activities comprised three domains: (1) legal activities, (2) residential activities and (3) family discussions about the future. Variables relating to one or more domains included whether the brother-sister lived in or outside of family home; brother-sister independent living abilities; presence of an intellectual disability; parent caregiving ability; and current sibling caregiving and involvement with the brother-sister with IDD. Although most families engage in some future planning, performance varies widely within and across future planning domains. Future planning involves different considerations and interventions depending on whether one is considering legal, residential or family discussions.