["Child &Family Social Work, EarlyView. ", "\nABSTRACT\nIn Western countries, childhood sibling death has long‐term health impacts, but this is little studied in China. This study aims to examine the effects of sibling death in childhood on the level, trajectory and progression of the frailty index in adulthood as well as the mediating role of parental physical maltreatment. Using data from the nationally representative China Health and Retirement Longitudinal Study cohort, a total of 12 738 participants were included. A four‐step mediation model with weighted linear and multi‐state models was employed. Experiencing sibling death in childhood was associated with increases in the level of frailty index and the odd of being in trajectory of rapidly rising to frail. Additionally, participants being exposed to sibling death in childhood were at a higher risk of progression from prefrail to frail. Parental physical maltreatment mediated the relationships of sibling death in childhood with frailty index level and trajectory. Our study extends the effects of sibling death in childhood to include its dynamic effects. A life‐course policy should be implemented, focusing on prefrail people. Given the mediation role of parental physical maltreatment in the associations in China, sibling death and parenting behaviour should be coordinated to maximize lifelong health benefits.\n"]