Pediatric-onset anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a group of systemic small-vessel vasculitides characterized by ANCA production. Primary AAV in children is rare and is generally considered to arise from immune dysregulation, environmental factors, and infections, with no strong evidence of familial clustering. This report describes a pair of monozygotic twin sisters who developed AAV concurrently. The elder sister, aged 9 years, presented with "anemia for over 20 days and abnormal urinalysis findings for 3 days" and was diagnosed with ANCA-associated vasculitis and ANCA-associated nephritis. The younger sister, screened via urinalysis following her sister's diagnosis, was found to have hematuria and proteinuria, leading to a confirmatory diagnosis. Notably, the sisters exhibited distinct disease subtypes, severity grades, clinical manifestations, and treatment outcomes. The elder sister's renal pathology revealed ANCA-associated renal injury with concurrent immune complex deposits, correlating with poorer renal outcomes. In contrast, the younger sister achieved complete renal recovery after treatment. Extensive exome sequencing failed to identify pathogenic variants in either case.