Non-classic Cystic Fibrosis (CF) still represents a difficult entity to diagnose. We present a case of two sisters affected by mild pulmonary symptoms started at puberty, carriers of the F508del mutation associated with the T5TG13 combination. We discuss the clinical utility of TG repeat testing in individuals carrying the T5 variant. Furthermore, this case-report leads to reflect on the natural history of CF and the correct management of its atypical forms.