I also use advantage on my cats and one time I had to bathe them because Starr had ringworm. I put advantage on them the next day and both Storm and Sunny had bad reactions. They lost some fur and it looked like they had hot spots on them. I had to use some antibiotic ointment on them for a while until things began to clear up again. I guess the bathing and advantage was too much for their delicate skin to handle.

I sure hope that Snow doesn't have ringworm. If your sister has it though then Snow may have given it to your sister. Please let keep us updated and I hope that things go well at the vets.