The "similar signs" bit doesn't sound good. It's my experience that FIP isn't contagious except by birth, and, even then, rarely. The 2 I had that got it were mother and son, although the rest of her babies that I've been able to follow did NOT get it. They lived here in the house with everybody else for several years, and NOBODY else got it either, not even MamaCat, who was Joy's absolute, inseparable best friend. Like you say, once Mist became sick, Joy followed very shortly thereafter in exactly the same way.