Leukemia is considered the most common childhood malignancy, and acute lymphoblastic leukemia is the most common type of leukemia diagnosed in children worldwide. The annual incidence has increased over the past decades. Thanks to improved diagnostics and treatment options, the survival rate has increased in all diagnoses in recent years. The diagnosis of leukemia affects not only the affected family member, but also parents and siblings. The relationship between siblings can be defined as one of the longest lasting relationships in an individual's life. Siblings of children with cancer go through major changes that affect their lives. A diagnosis of leukemia can significantly affect the quality of sibling relationships, as well as changes in psychological and social development. It can lead to changes in the environment of children's lives and in the family environment, but this does not necessarily have to be a negative experience. For a number of years, the sibling subsystem has been neglected and given little to no attention. Numerous studies have established that the perspective of siblings was viewed from the perspective of sick children and their parents. The opinion of the parents is woven into the opinion of the brothers and sisters. Recently, the quality of life has been highlighted as an important component. It is stated that the perspective of the quality of life of siblings of a child with a malignant disease is of great importance. More recent research has noted the difference between the perspectives of parents and siblings and allowed them to share their experiences related to the impact of leukemia on quality of life. The paper presents a qualitative research, the goal of which was to gain insight into the experience of the diagnosis and the way of coping with the diagnosis. The research was conducted through a semi-structured interview with the brother and sister of a child with acute lymphoblastic leukemia. The work resulted in positive effects of the diagnosis on siblings and the support systems needed for better functioning, which contributed to a better quality of life. Further research is needed that will focus on all aspects of the quality of life of siblings of a child with leukemia and on systemic support.