Adult siblings are potentially important sources of care, support, advocacy, and friendship for their brothers and sisters with intellectual and developmental disabilities (IDD). Drawing on data about 851 adult siblings who completed an online national survey, we examined predictors and potential key moderators of siblings' mental distress, wellbeing, quality of life, and health outcomes. Moderated regression analyses indicated that siblings experiencing higher levels of subjective poverty; siblings with brothers and sisters with lower levels of independence; and siblings who are carers and also experiencing low levels of subjective poverty, had worse outcomes and may be in need of specific supports. Compared to normative samples, adult siblings of people with IDD had worse outcomes across the measures.