Behavior problems and creative solutions:
Hissing at newly spayed or neutered kittens

Why do mothers and littermates hiss at newly spayed or neutered kittens? The reason that this happens is simply because the kittens smell like That Very Bad Place (the Vet's office). Once the smell fades, everything should return to normal.

To speed things along, you can do things to get rid of the vet smell. One thing is to brush the momcat and then the babies. Another is to rub a cloth on the momcat and then the babies. A third is to rub yeast flakes on the kittens (oddly, I do have yeast flakes in my kitchen). All of this will give them a more "pleasant" smell. The good thing about the third option is it will force the kittens to groom, which will make them smell more like themselves and less like the vet. The act of grooming also relaxes cats. If you don't have yeast flakes, you could try putting a little bit of food on their paws for them to lick off.