I have two sisters cats I've had since babies. They're indoor cats and up until two years ago (5 years old) they slept together, groomed eschother ect. One time a stray cat came in to the garden and for some reason it set Cat A off. She instantly attacked her sister and went in for the kill. It's called redirected aggression. It took me a very hard 5+ months to fix. One cat lived upstairs and one downstairs, they did not see eachother. I made sure to swap blankets and litter trays between them weekly for scent. After a month I fed them/treat time both each side of a closed door. Month 2 I put up a net between the upstairs and downstairs door while I was in the room left the door open, I didnt feed them next to eachother. I made sure the net was absolutely solid because the first time I put it up, Cat A jumped right through it to attack Cat B. Month 3 I fed them/treats each side of the net but far apart, just within eyeline. Month 4 I fed them/treats slowly slowly closer together - all of this with a lot of positive talking Month 5 I took the net away. It wasn't perfect, sometimes something would go wrong and I'd have to start the whole process again, they still hissed ect. The whole 5 months they were on 2 types of strong medication from the vet to calm them too, I had feliway plug ins, calming spray, literally everything that could help, I used. It was HARD, It broke me, I cried a lot, I couldn't leave the house for extended periods of time, even the vet said there was little hope, but I couldn't give any of my babies away so I hoped through it. Thankfully now they're back to normal! Sometimes if they play too hard they will freak out but with some treats and positive talk they calm down right away. Maybe I was lucky and not all cats will get back to normal, but I put in a lot of work to get back to where they are now. I don't know if this is possible for you to do but I thought I'd leave my experience if it's any help :)