Background The COVID-19 pandemic has caused many adults with intellectual/developmental disabilities (IDD) to lose their daily routines and social support, and as a result, many adults with IDD are increasingly reliant on their family caregivers. Siblings often play a crucial support role for their brothers and sisters with IDD. As such, this study aimed to describe the experiences of adult siblings of people with IDD during the COVID-19 pandemic. Methods The Sibling Collaborative worked with researchers to codesign an online survey, completed by 91 people, exploring sibling supports and concerns during the COVID-19 pandemic. The survey also aimed to identify helpful resources for siblings during this time. Results The results showed that the majority of siblings are supporting their brother or sister with IDD during the COVID-19 pandemic and are concerned about the health and well-being of their brother/sister. The most common concern related to disruption of their brother's or sister's routine and activities. Although responses of older and younger siblings did not differ from each other, siblings whose brother or sister with IDD lived with family had some unique concerns relative to those whose siblings no longer lived with family. Siblings described how their own self-care and relationships with others, as well as support for their brother/sister, were particularly helpful during the COVID-19 pandemic. Conclusions Siblings are providing key support to their brother or sister with IDD during the COVID-19 pandemic, and they too must be supported. Siblings should be included in efforts to disseminate resources targeting people with IDD and their feedback and input must be obtained. It is also important to include sibling mental wellness as caregiver supports are created and implemented. More research is needed to further understand how to support sibling caregivers.