Yes, FIP makes sense from your description.It is a complicated disease.
FIP is caused by a virus called a corona virus. There are more than one corona viruses that infect cats, and only one of them causes FIP, and even then not in every cat. FIP is a disease of young cats. What happens, is the cat is exposed to the virus. In most cats, there is some vomiting and diarrhea
and the cat gets well and that is the end of the story. In certain cats (and we can't tell which ones), the virus mutates once inside the cat into a different virus, the FIP virus. Once it mutates, it gets into the cat's immune system, and turns the immune system against the cat, and causes FIP. Sometimes FIP appears as a big belly of fluid, but sometimes the fluid builds up in the lungs, and sometimes there is no fluid at all, but the cat gets kidney
failure, or neurologic disease. However FIP infects the cat, it is always fatal and incurable.
The good news is most cats do not get FIP from the corona virus, and survive and live a normal life. There is no good test. You can test for Corona virus, but if the test is postive, it does not mean the cat will get FIP; the odds are it won't. It just means the cat has been exposed to a Corona virus.
Your cat has a decent chance. I have seen litters of kittens, where a couple got FIP and died, and the rest were fine. All you can do is watch for symptoms such as vomiting, diarrhea, coughing, loss of appetite, etc. Have your kitten checked by a vet, and tell your vet the litter mate died of FIP. I hope your kitten is one of the lucky ones. Hope this helps, Rebecca