Siblings of a child with a life-threatening disease, such as cancer, have a right to measures that promote their health and welfare. Siblings may find it hard to understand what is happening to the sick child with cancer and why he or she reacts as he or she does. The aim of the study was to explore sibling supporters’ thoughts about the experiences they had in providing support for siblings with a brother or a sister with a life-threatening disease such as cancer. All the 12 sibling supporters currently working in Sweden participated in a qualitative, descriptive study from which 5 categories emerged, showing that the sibling supporters supported siblings from diagnosis until possible death. They enabled siblings who were in the same situation to meet each other and arranged activities suited to their ages, as well as offering an encouraging environment. To help the siblings, the sibling supporters found it necessary to interact with both the parents and the ward staff. The sibling supporters felt that their support was important and necessary in helping siblings promote their own health both when the sick child was alive and also after his or her death. The experience of the sibling supporters was that they listened to the siblings’ stories and met them when they were in their crisis. The study confirms that sibling supporters should be a part of the health care team that treat and support the family when a child has cancer.