There is never a 100% guarantee in anything you do related to CF and IVF/PGD that will ensure a child without CF. I know a family who has a DD with CF. They did IVF with PGD last year to "ensure" a child without CF. They know what their mutations are, easily identifiable... Their twins were born about 2 months ago. One of their new babies HAS CF. After PGD and everything. So, there are NO guarantees.
But to answer your question, YES, waht you told them is right. There are "nonsense" mutations that can potentially have a lesser effect on the overall person. But then again, they are still doing research regarding mutations and degrees of CF. Some siblings in the same family (who will then have the same mutations) have varying degrees of CF.
In my opinion it is irresponsible of your sister to think this way and she should really furthe educate herself before she makes a choice like that. Now, if she's educated and understands and still makes the choice, that is her choice, and not irresponsible in my opinion (to each his own I say), but she should really get all the information before making a choice.