There is information on this page about FIP hope it helps you..  

http://www.tica.org/vet.htm

Aimee

FIP is fatal, no cure, no treatment.  Typically happens in kittens under 18
months of age.  Stands for feline infection peritonitius.  Extremely hard to
give definite diagnosis.  They can do extended labwork which basically can
tell you if the cat has been exposed to the corona virus - FIP is a mutation
of the corona virus.  80% of all cats have been exposed to the corona virus
(if cats are around dogs that have been vaccinated against corona, the cat
has been exposed to it).  Exposure does not mean the cat has FIP.  A high
globulin count can indicate FIP (which would be indicated in bloodwork).
They could have a temperature as well.  They run two titers typically, can't
remember exactly what they are.  I've heard if the lab can run a 1:25 titer
(most won't/can't) and it's positive, the cat more than likely has FIP.

There are two kinds of FIP - wet and dry.  The wet from what I understand is
easier to diagnose, I think the belly can start filling with fluid. The dry
form dry granuals on the internal organs.  Typically way to tell for sure is
by doing necropsy after suspected cat has passed away.

From what else I've read is it's not transmitted through body fluids, rather
thru feces. If you suspect FIP and have other cats you need to keep those
boxes CLEAN, empty frequently and clean box with bleach mixture.

I just went this with one of my cats - it's extremely frustrating, you walk
out wondering what to do.  I was told bloodwork was within normal limits,
had no temp, but the titers both were positive - meaning she had been
exposed to the corona virus.  The vet told me she found 11 month old stray,
wasn't gaining weight as it should, she kept testing, finally noticed one
pupil was slightly larger than the other, that was how she determined it was
FIP, the cat went downhill within weeks.  She told me if my cat did have FIP
I would see the same quick/fast deterioration in health.  That was a little
over a month ago, the cat has since improved so I very much doubt at this
point if she has FIP, whatever it was is out of her system and she is a fat
and happy cat.

Do you think the kittens have FIP?  Are they acting sick?  If they are, it
would not hurt to go to vet and get bloodwork.  You should always keep new
kittens away from other cats for a few weeks, at least until you know they
aren't carrying Felv or FIV (feline aids - which attacks the immune system).
And do not let them share litter boxes with your current crew until you are
sure they are healthy.  If the kittens aren't showing any health problems
the blood test isn't going to tell you anything as far as FIP goes.  Sorry
to paint a grim picture - FIP is a terrible disease, made all the more so
because there is no definitive diagnosis. There's also a very small chance
cats have it (very low percentage).  I would still recommend vet check
anyway as they are feral.  They will more than likely need treated for
worms.

Best of luck.

JoJo