Hi,
I know a friend in Japan, whose younger brother and sister have CAH (actually, her sister had CAH, since she died when she was three from adrenal crisis). My friend is a carrier, but didn’ t know that until she had her daughter affected by CAH. You can imagine how shocking that was for her.
In my very distant relatives, a baby with CAH was born last year (by the way, I have three year old son with CAH). I don’t exactly know the baby’s relationship to us; something like my grandfather’s uncle’s great-great-grandson.
Although I know these examples, I agree with the previous posts. There is only a tiny chance for your niece to have an affected baby. Perhaps you can just suggest her to be watchful for any signs of CAH after the baby’s born.