Loss of sibling relationships is a common experience across international jurisdictions for children entering public care. This is the case despite statutory guidance that emphasizes the need to place siblings together when in their best interests, and increasingly robust evidence of the protective nature of sibling relationships when children face adversity. Research on the experiences and outcomes of siblings in care has thus far focused predominantly on placement and contact patterns, particularly of siblings in care concurrently. This study extends this research by comprehensively mapping sibling networks both within and outside the care system and measuring sibling estrangement (living apart and lack of contact) over time. Drawing on administrative and case file data within the Children's Hearings System in Scotland, the circumstances of 204 children and young people from 50 sibling networks were examined longitudinally. The study found very high rates of sibling estrangement with seven in 10 relationships between a child in out-of-home care and a sibling classified as estranged and half of all siblings classified as strangers (siblings having never lived together and no record of any communication or meetings between the child and sibling). Moreover, sibling estrangement increased significantly as children moved through the care system. We argue that continued effort is needed to improve the accuracy with which aspects of sibling relationships of children in care are recorded and measured in order to assess the longer-term impact of state interventions on children's lives and the capacity of child welfare agencies to meet policy goals.