FIP is highly transmittable. I'm not sure of the incubation/testable period...but I think it is under two weeks. I would say to get them tested about...well "Now", as it's been some time since the death? I'd also replace the litter box and food dishes to be safe (if all are plastic).
Don't lose hope, it does take direct saliva/blood contact for transmission. So if there's been no play/fighting that drew blood, that leaves litter box/food dish transmission, which from what I've seen is not very high risk transmission danger.
Sorry for the loss of your wee one.
I know what you are going through, we had to put our cat down because he was diagnosed with FIP - the wet form. I do not know how long she had FIP to know what kind of risk your other cats are in. I know that the veterinarian told me it's contagious. I would go get your cats tested, from what I heard, testing is kind of hard because even if a test comes back FIP Negative, that doesn't mean that in the future, it won't come back as FIP Positive or vice versa. That's at least what the veterinarian I went to told me.