The aim of this study is to investigate behaviour problems and emotional regulation of children who's siblings are diagnosed with cancer. Participants were assessed with the Strengths and Difficulties Questionnaire (SDQ) and the MacArthur Story Stem Battery (MSSB). Both instruments were administered twice (shortly after diagnosis and at the end of treatment). Brothers and sisters of 14 children suffering from cancer were compared with 18 siblings of healthy individuals. In the SDQ no differences between the two groups were found. At time 1 the MSSB showed significant differences in number and quality of displayed contents, representation of parents and the way of performing the narrative. At time 2 narratives changed particularly in the treatment group, but some differences still remained. Overall siblings of children suffering from cancer showed more intrapsychic reaction to the disease than became obvious in their behaviour. This fosters the conclusion that there may be a need for specific support for siblings of children with cancer in order to prevent them from psychic disorders and pathological mourning reactions.