Background: Children of parents with cancer are at risk of developing mental disorders. Results from divorce research also reveal that sibling relationships can protect the mental health of children in difficult times. Objective: Does having a sibling help to cope with an oncological disease of a parent and thus act as a protective factor? Methods: A group of 271 children were examined in a multicenter study. 54%made use of the offered psychosocial support. Only children (N = 89) and children with siblings (N = 182) were compared with respect to their mental health (Strength and Difficulties Questionnaire, SDQ, parental and self-assessment). Results: The group comparison between only children and siblings showed no significant differences in the SDQ (assessed by healthy/ill parent). In the self-assessment 2% of the only children and 9% of the siblings showed significant results on the SDQ. The group comparison between only children and children with siblings failed to reveal any important differences in mental health. The study indicates that a negative relationship quality (Sibling Relationship Questionnaire, SRQ) is associated with increased problems in the peer group. Conclusions: The existence of a sibling is not per se a protective factor. Only children do not show more signs of emotional stress than children with siblings.