When Flobby was eventually allowed home, Sputnik's attitude to her 'brother' changed dramatically, she hissed and growled and would have nothing to do with any of us. We have tried to show equal affection to both and are now afraid to leave the two alone in case Sputnik attacks Flobby whilst he is in this delicate state. She will also have nothing to do with me and continues to lash out if I go near her. I love them both dearly. Dear Human, Sputnik is rejecting Flobby because she does not recognize him.
Cats recognize each other by smell.
While Flobby was at the vet he picked up a lot of strange smells and no longer smells like his old self. I suggest that you separate the two cats for a while to let Flobby recover and to give Sputnik time to learn to recognize him again. The cats can visit under the door. Every day, switch the cats around for a few hours and let them get used to the smells of the other cat. It shouldn't take long before your kitties are best friends again.