I've been pondering a similar point myself. I read awhile back about this couple who happened to be deaf purposely choosing a deaf man to have a child with so that the child would be deaf, too. I don't think that was ethical of them.
But on the one hand, in the scenario you mentioned, it is possible that the lady thought her child wouldn't get the disease if her husband's family doesn't carry it. I can see how she would think that, and even so: everyone has some sort of time bomb in their genes. Everyone. So the idea of maybe passing something on, and therefore deciding not to have a child isn't so persuasive as it might seem at first. The lady might not be so kooky in that light.
But on the other hand...! Right now people have the option of selecting to have a girl or a boy, and I'm wondering if down the line your coworker's sister wouldn't be legally required to make use of that technology when it becomes more prevalent.