I'm curious if the vet discussed the possibility of cerebellar hypoplasia aka Wobbly Bobbly syndrome. What you're describing doesn't completely fit, but partly fits. If the mom cat has had distemper, all the kittens in the litter can have this where their balance is so bad they have trouble walking and constantly fall down. It's benign, and while it doesn't really get better, although they can learn to compensate some, it doesn't get worse either. Cats don't die from it so the sad news about one of your kittens dying doesn't fit with the syndrome.
In any case, I thought it was worth mentioning because its not all that uncommon to see in rescued cats where the mom is more likely to have had distemper, but still rare enough that not many people know about it. I've seen several cases of it at our rescue group, we even have an adorable kitten with it now. Whenever you have multiple kittens from the same litter with balance problems its worth considering.http://www.vetinfo.com/cencyclopedia/cecerhypo.html