Were the cats vacinated by a vet? Feline aids and leukemia can lie dormant for a while and then kill a cat in short order. They’re also very contagious. Your other cat may be in danger of death from the same disease. It’s also possible that your little cat got into something they shouldn’t have and ate/drank something toxic.
I had one of my cats die several years ago from kidney failure. The vet suspected that he drank something like antifreeze but he was an indoor cat and I never had anything like that in the house (cabinets had child/cat proof locks on them). Later we remembered that this cat liked to lick the water out of the shower in the morning and it turns out that our well water was contaminated and that was what poisoned him (we used bottled water for cooking and drinking, including the cats water bowl).
I feel bad for your loss but you need to try and figure out what happened before the other cat gets sick too. Currently I have four cats and I religiously keep my house free of anything poisionous or dangerous to them.