One cat with FIP, three others in the house!

This might be long, but i’m specifically asking advice from those who have experienced FIP in their homes.

I live with my girlfriend and her families cat was found out to have FIP today. He is staying with the vet to do treatment while the family is out of town. For the past two-ish weeks he’s been isolated because the vets originally thought it was an anemia illness, he’d been on antibiotics and what not, but today they learned it was FIP

We moved into the house in February with our three cats. My two cats, girls over the age of 2, always left their cat alone. Our boy cat, under one, who was adopted with the cat with FIP, would always try to play and fight with him. We originally thought our boy cat may be a dormant carrier of the anemia illness, but of course our theory has changed.

We use Dr. Elseys litter, and i change my litter frequently as my litter boxes are the most popular in the house. They haven’t been used by the families cat in weeks because of his isolation.

To be preemptive, does anyone have any advice? How do I protect my babies best?