Cystic fibrosis (CF) is a progressive disease with no known cure. Advances in diagnosis and treatment have resulted in patients living longer and thus families live with the illness for longer. Treatments are becoming increasingly demanding and are largely performed in the family home. Mothers are often reported to experience greater stress and poorer adjustment than mothers of well children or population norms. Patients and siblings are also reported to display adjustment difficulties. Siblings have rarely been included in research designs. This qualitative study investigates the impact of CF and treatment on eight patients, eight mothers, one father and eight siblings. A family systems perspective was adopted. Each individual was interviewed independently using semistructured interviews. Patients and siblings were aged between 9 and 21 years. Qualitative analyses revealed high levels of non-adherence (intentional and unintentional) and parental involvement in treatment, minimal involvement of siblings, and preferential treatment towards patients. Demanding treatment, coupled with the progressive nature of CF, promote high levels of parental involvement for younger children as well as older teenagers, often due to attempted or actual non-adherence. Siblings may receive less attention while patients' needs take priority. Future development of a measure of adherence suitable for children and adolescents should take into account different motivations for non-adherence, particularly regarding the level of personal control over adherence to treatment. In addition, the potential impact of having a brother or sister with CF should not be underestimated and the needs of siblings should not go unnoticed.