Agree about keeping the family together. As my vet would say, they have
already been exposed.
I would separate the other cat for sure, at least until she's had the
booster (30 days?). She is not protected right now. Wondering how long they
have all been together. In any event, I think I'd keep her apart now and
until everyone tests negative. I'd test her again, too (I can't remember how
long a wait is recommended before retesting).
I am questionning why each cat isn't being tested before they are
co-mingled? We always tested each cat before co-mingling.
In my own home, I did test and vaccinate and booster each cat who joined the
family and whom I fostered., Any new cat was isolated for 2 -4 months, until
tested negative twice, and vaccinated and boostered before meeting everyone