DR Cilla, Phoebe is very pretty.  I agree an allergy test to cats was in order.  They could have been sick from anything, including something that was on her.  Next time Sam & her mother visited your parents maybe they can give Phoebe a bath in an unscented gentle soap & then rinse her well with distilled water.  That has always helped my allergies to cats since I'm really allergic to what gets on them, not to their dander.