Background Allogeneic haematopoietic stem cell transplantation (HSCT) offers a potential cure for a variety of diseases but is also associated with significant risks. With HSCT the donor is either a relative, most often a sibling, or an unrelated registry donor. Purpose The aim was to explore patients’ experiences, immediately before transplantation, regarding having a sibling as donor. Method Ten adult patients with sibling donors were interviewed before admission for HSCT. The interviews were digitally recorded, transcribed verbatim and subjected to qualitative content analysis. Results The main theme Being in no man’s land is a metaphor for the patients’ complex situation with its mixture of emotions and thoughts prior to transplantation. The three subthemes Trust in the sibling donor, Concern about others and Loss of control cover the various experiences. The patient’s experiences are influenced by their personal situation and the quality of the relationship with the sibling donor. While patients feel secure in having a sibling donor, they are dependent for their survival on the cell donation and feel responsible for the donor’s safety during donation. These emotions intensify the patients’ sense of dependency and loss of control. Conclusions In caring for HSCT patients the nurses should be aware of the complexity of the patients’ situation and keep in mind that having a sibling donor might imply extra pressure, including a sense of responsibility. Caring for both patients and sibling donors optimally is a challenge, which needs further improvement and exploration.