Something like 1 in 25 or so people are carriers. I'm not sure on the exact #, but it's pretty close to that, depending on ethnicity. If your son in law is a carrier, then every child they have has a 25% chance of having cf, 50% chance of being a symptomless carrier (like your daughter), and a 25% chance of NOT having cf or carrying the gene. If that's the case, they should discuss it with a genetic counselor who is really current on cystic fibrosis. Also, keep in mind that even if your son in law tests negative, there is still a chance that he could be a carrier of a rare mutation and therefore, their children could still possibly have cf. Only the Ambry genetics test screens for all known mutations and it is not the most widely used test, especially not for genetics counseling/prenatal testing. Also, if your other daughter's children have ANY symptoms of cf (lung problems like RAD or asthma, recurring lung infections, serious sinus problems, failure to thrive or gi problems, recurring infections), they should be tested. If they don't have any symptoms, your other daughter may still want to be tested for the same gene your daughter has. If she also has it, she may want to have all of her children sweat tested just to be on the safe side. What gene does your daughter have? HTH.