If sophie has a brother or sister from the same parents what are the chances that sibling would have cystic fibrosis?

There would be a 20 % chance that the siblings would have cystic fibrosis from a series of calculations. The primary cause of cystic fibrosis is when the CFTR gene has abnormalities. This specific gene regulates the influx of salt and water in our body, thus when a person has this disorder, he or she may have a very salty sweat. 

25% chance that the sibling has cystic fibrosis

The answer is c) tons