Background: Adult siblings are important in the lives of individuals with profound intellectual disabilities, especially as parents age. However, little is known about the roles they assume. Method: We examined these roles among 58 participants from the Netherlands, who completed an online questionnaire. Results: Most participants (89.7%) assumed multiple roles, and the majority were content with their roles. Between 7% and 58% indicated that they were the only individuals providing a certain role. Shared roles primarily involved healthcare professionals, other siblings, and parents. Almost half the participants (48.2%) assigned higher scores for the amount of support provided than for the experienced burden. Conclusions: Adult siblings often assume multiple roles. The shared roles emphasise the importance of collaboration.