Adult siblings of individuals with intellectual and developmental disabilities (IDD) report struggling to navigate the adult disability service delivery system and collaborate with professionals. To date, though, it is unclear how professionals encourage sibling involvement and, accordingly, the facilitators and challenges in working with siblings. For this study, 290 professionals participated in a national web-based survey; participants answered three open-ended questions about ways to involve siblings, positive experiences with siblings, and challenges in working with siblings. Professionals reported person-level and systems-level supports to encourage sibling involvement. Also, professionals reported enjoying working with cohesive families of individuals with IDD and witnessing the benefits that siblings bring to their brothers and sisters with IDD. Challenges in working with siblings included: lack of sibling involvement, systemic barriers, and caregiving burden. Implications for future research and practice are discussed.