I had a similar experience, although it wasn’t quite as sudden as your sister’s cat. About 10 years ago I had a cat that had what was thought to be seasonal allergies. He would be fine through the winter and then start coughing in May. When he started coughing I would take him to the vet for a steroid injection and then he would be fine for a time. The last May when he started his coughing episode I took him to the vet as usual. The vet listened to his heart and lungs and didn’t notice anything abnormal, so he gave the cat a steroid injection. The cat had been stressed from the vet visit, and on the way home he was panting. I thought it was a reaction to stress and that he would be ok once he was home. It didn’t get better, and in fact in got worse. Long story short, when I took him back to the vet he was in crisis and I was frantic to help him. They x-rayed his chest and he had fluid constricting his heart. The vet wanted to try to get him through this until they could get a diagnosis, but the cat died that night. I gave permission for an autopsy, because I wanted to know what happened. The cat’s lungs were 80% scar tissue, and the final diagnosis was feline infectious peritonitis. His lungs had been deteriorating for a couple of years, and the stress from the vet visit just triggered the final episode.
Kyrabee, very likely your sister’s cat had an underlying undiagnosed condition, and the stress from the vet visit triggered it. If that’s the case, nothing could have saved him, and in fact it may have been better that he was at the vet where he was immediately treated and didn’t suffer very long. It was just an awful thing to happen.