This chapter explores the phenomenon of sibling abuse through the lenses of systems theory, social learning theory, trauma theory, and feminist perspectives. Although sibling relationships are among the most enduring and influential within family systems, their potential for harm has been historically overlooked in scholarship, policy, and practice. The chapter identifies four major types of sibling abuse—physical, emotional, sexual, and financial—and situates them within broader family dynamics and intergenerational cycles of violence. Systems theory highlights the interdependence of family members and the role of unresolved parental conflict in fostering abuse. Social learning theory emphasizes the modeling of aggression and maladaptive behaviors within families, particularly in households marked by domestic violence. Trauma theory illuminates how exposure to abuse disrupts attachment, emotional regulation, and long-term psychological health, while feminist theory underscores the role of patriarchy and gendered power imbalances in shaping patterns of victimization. Case studies and empirical research demonstrate the widespread prevalence and devastating effects of sibling abuse, including its frequent underreporting due to stigma and family loyalty. The chapter concludes by critiquing theoretical gaps, calling for integration across frameworks, and urging more comprehensive interventions and policy responses to address this neglected form of family violence.