AIM: This paper is a report of a study of the process of grandparent involvement with siblings of preschool children with hypoplastic left heart syndrome. BACKGROUND: An increasing number of grandparents are involved in parental or near-parental roles with their grandchildren. Most research concerns grandparent involvement due to parental issues (e.g. teenage pregnancy, mental illness, addiction). Some research addresses grandparent involvement when their grandchild is ill. Grandparents”double concern’ for both their adult children and their ill grandchildren is reported in the literature. In this paper, we describe a third concern for grandparents: the sibling(s) of their sick grandchild. METHOD: Individual interviews were conducted in 2007 with 15 grandparents of six preschool children with complex congenital heart disease. Open and selective coding, categorization, and theoretical memoing were used to analyse the data. FINDINGS: ‘Stepping in as needed’ and ‘safeguarding relationships’ were identified as two core categories related to grandparenting siblings of children with heart disease. Grandparents stepped into a parent role with toddler and preschool-aged siblings by attending to their daily care routines, recreational and play times, and relational needs while parents were occupied with their sick and hospitalized infants. Grandparents’ concerted efforts to sustain parent-child and child-sibling relationships were also striking. CONCLUSION: Our findings extend the concept of ‘double concern’ to ‘triple concern’, and direct a research and practice focus towards the unexplored roles and needs of grandparents and siblings in families whose young children have life-threatening illnesses.