The authors present a case of Fragile X syndrome (FXS) in siblings from an Indian family with no developmental delay in previous generations. The boy presented with developmental delay, autistic features and defiant behaviours that raised clinical suspicion. He also had congenital diaphragmatic hernia (CDH). Social anxiety and difficulty in making friends were the subtle features in his sister with dull normal intelligence. FXS was confirmed by clinical features and DNA testing. Intervention was initiated for both the siblings. Screening siblings in FXS is important. CDH can be associated with FXS.