Introduction: Siblings of children with cancer experience psychosocial distress during the illness and after bereavement, but often stand outside the spotlight of attention and care. This study explored bereaved siblings’ advice to health care professionals (HCPs) working with children with cancer and their families. Materials and Methods: In a nationwide Swedish survey of bereaved siblings, 174/240 (73%) participated. Of these, 108 answered an open-ended question about what advice they would give to HCPs working with children with cancer and their families. In this study, responses to this single question were analyzed using content analysis. Results: The most common advice, suggested by 56% of siblings, related to their own support. One third suggested giving better medical information to siblings. Some siblings wanted to be more practically involved in their brother’s/sister’s care and suggested that HCPs should give parents guidance on how to involve siblings. Other common advice related to psychosocial aspects, such as the siblings’ wish for HCPs to mediate hope, yet also realism, and the importance of asking the ill child about what care they wanted. Conclusion: Information, communication, and involvement should be emphasized by HCPs to support siblings’ psychosocial needs in both the health care setting and within the family.