Try a dab of butter
The smell of the vet office used to be enough to get my other cats hissing when one returned....that is normal.
Most cats love butter, so a little dab of butter on each of their foreheads, or on back of their necks should get them into mutual grooming. It takes a while for the homones to simmer down, but in a couple of weeks things should be back to normal. Let us know how these two sisters progress.