Hi, Carriers, as far as I know, do not have CF symptoms. This I know though, You either have CF or you don't. There is no "full blown CF". It is however possible to have CF and not ever know it. There seems to be a difference in some CF mutations that cause some CF'ers to have very little or no lung involvement and even very little to no GI involvement. To make myself a little clearer, there are some people that were not diagnosed until adulthood, some I have heard of not diagnosed until their 40's and 50's. Not knowing all those years that their chronic sinusitus or chronic asthma was really CF. There seem to be many adults these days getting diagnosed with CF.
Should she be tested for CF? That's a tough one to answer. In my personal opinion it really depends on what is going on with her health. It's really a loaded gun. There are so many things to consider even just being tested and then if the test comes back positive. My husband and I have discussed just for conversation, if you were an adult with no real health problems, say 30,40 yrs old, would you want to be tested and get a whole set of possible problems, from being denied health insurance to life insurance, the added worry about every little sickeness? It's a tough one to consider. But again, in my opinion, if you are a healthy person. By all means if you have some unexplained chronic health problem, whether it be asthmatic or GI, then yes, I would think so.
I guess it depends on what CF symptoms she is showing. Chronic sinusitis, GI problems, allergies, asthma, chronic pulmonary infections, foul gas/BM, greasy, pale, floating BM? Any, all, some of these together? Again, it's a tough personal decision depending on her health.
Just in case you didn't know yet though, both parents must be carriers to pass on two CF genes. If both are carriers there is, they say, a one in four chance of each passing on a CF gene. If one parent is a carrier and the other is not then they can only produce a carrier or a non-carrier, If one parent is a carrier and the other a CF'er then they produce either a carrier or a child with CF. And last of all if both parents have CF their child will too. I forgot,to mention, two carriers can produce a non-carrier too.
I hope that I have been of some help. I hope that all goes well with your niece's child and that he/she does not have CF. My advice first though would be for her to wait until the results of the sweat test for her child come back, then go from there.