Pedigree — Somatic Mosaicism: McCune-Albright Syndrome (Post-Zygotic GNAS Mutation)

A 4-year-old girl presents with vaginal bleeding, breast development, and unilateral café-au-lait macules with jagged borders on her left trunk. Imaging shows polyostotic fibrous dysplasia of the left femur with a characteristic "shepherd's crook" deformity. Genetic testing of blood lymphocytes is negative. The structure marked **A** in the diagram represents the genetic mechanism underlying this presentation. Which of the following BEST describes why this patient's siblings have essentially NO recurrence risk despite her having McCune-Albright syndrome?