Purpose <p>Psychosocial support for siblings has been identified as a standard of care in pediatric oncology. The aim of this study was to explore health practitioners’ perspectives on how siblings of children with poor prognosis cancers are supported.</p> Methods <p>Using a purposive sampling strategy, we interviewed 14 oncology and palliative care health practitioners at a Canadian tertiary pediatric oncology center. Our data analysis used Interpretive Description. Semi-structured interview transcripts were coded inductively and refined using our theoretical framework.</p> Results <p>We identified four themes that influenced health practitioners’ ability to provide psychosocial support to siblings: (1) individual siblings’ needs and goals, (2) parents’ role in protecting their children, (3) realities of practice for health practitioners, and (4) systemic factors influencing sibling support. Within these themes, we identified challenges to psychosocial support for siblings of children with poor prognosis cancers.</p> Conclusion <p>Our study highlights barriers to sibling support prior to bereavement and opportunities to improve care. Based on our findings, we developed recommendations to address these barriers. Oncology programs can use these recommendations to reflect on and improve sibling support practices.</p>