Hello,
I am sorry about your other kitty, and that your kitty is now sick. It is impossible to tell you what is going on based on the symptoms and history you provided, but I can tell you with certainty that your kitty is very sick, and needs to see a vet IMMEDIATELY!! She needs to be seen right away, so if your regular vet is not open when you read this, you will need to tak her to the nearest emergency hospital. It is VERY dangerous for a kitty to go longer than 12 hours without eating or drinking, because the liver and kidneys have very specific nutritional requirements, and can start to shut down without adequate nutritional support. Plus, your kitty is also becoming dehydrated.
Sadly, your kitty may have FIP, or she may have leukemia, or even feline aids. It is also possible that something else altogether is going on. To get some answers, she needs to be seen by a vet for an exam and some blood tests, and x-rays. To save her life, she needs to be seen immediately. The longer she goes without medical attention, the more dehydrated and decompensated she is going to become, and the more difficult and expensive it will be to make her well again. Please rush her to the nearest emergency hospital right away and let me know if I can be of further assistance. If this has been helpful, please hit the green accept button. Good luck!
Emergency Critical Care Nurse
16+ years of veterinary experience