My 2 Year Old Grand Daughter Has Been On Antibiotics

My 2 year old grand daughter has been on antibiotics her entire life for sinus infections. She had her adenoids taken out and now they are going to take out her tonsils. Today, the doctor said he needed to test her for cystic fibrosis. Why???? She doesn't have a persistent cough or thick mucus in her chest - her chest is always clear it is just her sinuses. How could this be a possibility?