I would say it's not really necessary no. If you're prepared for the fact that the baby might have CF, then there's no extra real precautions to take or anything, if your daughter is going to go ahead with the pregnancy regardless. I am sure the chance of harming the baby and also I believe there is a small chance of making mother infertile through testing in the womb, would be of bigger concern than maybe having a child with CF.
Has the father been tested? Testing him would at least give some indication of whether there is even the chance that her baby could have Cf. You need 2 carrier genes, one from each parent for their child to have CF, there is a 1 in 4 chance if both parents are carriers.
The good thing is, with your daughter testing positive for the carrier gene, at least they know to crack on straight away with testing off the little one when it arrives. Heel prick tests have only recently started to be used on every child. When your daughter was born they will have relied on a sweat test if their were any symptoms and also not many doctors had even heard of the condition. I had problems for 2 years before someone finally came along and noticed the symptoms in me, and I'm 25. If your other children are around this age or older, chances are they won't have been tested at birth and not having any problems to suspect CF, then testing won't have been done on them. Your other children should get tested to see if they're carriers too so when they're starting families the risk of having a child with CF can be avoided.
I hope it is a smooth pregnancy for your daughter and also that eveything turns out good.
All the best
Hey, I'm Gem with CF, age 25 and diagnosed at 2 years old. I'm from the UK and say weird things sometimes. I'm no expert but I've lived with a lot of CF.
Diagnosed with: Cystic Fibrosis, Asthma, ABPA, Clinical Depression, Mild Liver Cirrohsis, Mild Osteopenia, Erythema Nodosum. Waiting for final diagnosis on Muscular Dystrophy type thing.