Objective: Familial Mediterranean fever (FMF) is a hereditary, autoinflammatory disease. The characteristics of siblings with FMF have not been described in large cohorts up to now. This study aimed to examine the features of siblings with FMF. Materials and Methods: This was a retrospective, cross-sectional study. Patients were divided into 2 groups according to the time of diagnosis (group I, the child diagnosed first in the family, and group II, the sibling diagnosed later). Results: A total of 143 siblings (65 families) with FMF were included in the study. Seventy-two percent of the patients had the same genetic mutation as their siblings. Despite having the same genetic mutation, 59% of the patients had different attack symptoms from their siblings. In 56% of the patients, the Pras disease severity score and in 45% of the patients, the response to colchicine treatment differed from their siblings with the same mutation. Fever and abdominal pain were statistically significantly more frequent in group I than in group II (P = .032). The age of disease onset in group I was statistically lower than in group II (P = .031). Genetic mutations, attack symptoms, and colchicine response were the same in twin pairs. The age of disease onset and age at diagnosis were also the same in half of the twin pairs. Conclusion: Parents of children diagnosed with FMF should be informed of all the symptoms of FMF disease and that siblings may present with different clinical findings.