Objective: This study developed a comprehensive framework for understanding the process of sibling adjustment to pediatric acquired brain injury (ABI). Participants: Grounded theory methodology was employed to inductively explore the issues siblings perceived to be their main concerns and how they managed these concerns. Fifty-three interviews (N = 28) were conducted recursively with 20 child and adolescent siblings of individuals with an ABI, 4 adult siblings of individuals with an ABI, and 4 child and adolescent siblings of individuals with congenital disability. Observational and secondary data from hospital staff and parents were also analyzed. The framework was developed and verified through simultaneous data collection and analysis that continued to the point of data saturation. Results: The main concern for siblings was the loss of equilibrium in their lives as defined by the concepts of vulnerability and emotional turmoil. Losing equilibrium was enduring and threatened siblings’ sense of security, safety, predictability and control many years post injury. To manage these concerns, siblings employed a variety of interrelated strategies to regain equilibrium. These strategies were conceptualized by the concepts navigating and sacrificing to restore safety, predictability, and control. Like the nonfinite nature of losing equilibrium, regaining equilibrium was an ongoing cyclical process. Conclusions: Rather than focusing on adjustment outcomes only, this study extends previous research by offering a framework for understanding the process of sibling adjustment, and thus providing a set of integrated categories, concepts, hypotheses, and propositions to inform future research and practice.