Hi, Tayslin1 and Frhnshrn,
I found this article on the web site of the Cornell University Veterinary College. (Where my dad studied, actually. :-) It's well-written, not overly technical, and it answers a lot of the common questions, including the one you are both asking: "We keep our cats indoors, and they're vaccinated, so why do they have FIP??"
The good news is that it's not easily transmitted to other cats. Unfortunately, this article doesn't provide a miracle cure, tho. :-(
http://www.vet.cornell.edu/fhc/brochures/fip.html
Wishing all the best to you and your cats...