Yeah it is all a little crazy huh?
I'm not so sure about other genetic diseases, I'd guess so. Especially the liver as that is faster to rejenerate than lung tissue. I think the lungs is still a slow process as once the CF genes are back in place, things will be different the second time around, as treatment is better, and you are more aware of infections, than you are in childhood. We all used to play together as kids, there were no cross-infection rules, meaning mutations of infections were made that thrive only in CF lungs. The longer you can avoid CF infections, the longer it will be to have any significant lung damage.
The good thing is-when I was born, my life expectancy was 5, I beat that. I beat the 12 year mark and the 21 year mark. I'm due to beat the 25 year mark in February. Up until a couple of years ago, the expectancy was 31, now it's 40 I think!! This is good news though, as it means by the time your little cousin reaches my age, life expectancy is probably going to be normal-at least 60! Even without a cure coming a long, this is where we are headed. Treatment is a lot better now than what it used to be. I think when I was diagnosed at 2, I'd just missed out on the previous dietry advice of eating nothing with fat in it, as enzymes weren't all that good.
Now treatment is there for mucus thinning, better antibiotics, enzymes. It's all just generally looking better all round. There is now Dornase Alfa, which is a nebuliser that breaks down certain DNA cells in the lungs which is what makes mucus about 10 x thicker for us. This is pretty new, and I'm sure people taking it before lung damage really is a problem, will help prevent infections in the younger generation.
I'm not saying there is nothing to worry about, I'm just saying there is now a lot less to worry about. Perhaps when your cousin gets to their 20's, which is usually when lung transplants are happening now, it might be that it isn't even thought about until a lot later on, as treatment is always advancing.
CF is still pretty young, in how long it's been known about, and even how recent the location of the genetic fault itself has been.