Petrie, I’m so sorry you lost your cat. It is such a devastating disease, and I have lost several cats over the years to it. Statistically, something like 80 to 90 percent of cats have been exposed to the corona virus, so the fact that the sibling tests positive wouldn’t be alarming to me.
I have a senior cat whose sibling died of FIP before he was a year old. This cat has never had any issues.