Q. My cat hisses at people coming in the house, especially the cat sitter; what can be done
A. Your cat could just be exceptionally shy around people she doesn't know well. I've done pet sitting at client's homes where I only catch a glimpse on one of their cats while I am there. Make sure the cat has plenty of places to hide and get up away from anything that may frighten her. If this is a problem because the pet sitter has to medicate her, when you leave you may have to close doors leading to bedrooms and other places where she can hide so that the pet sitter doesn't have to search every corner of every room to find her. I know, because I've had to do that myself. Just optimize the environment for your cat all the time, so that she feels as secure as possible. If your pet sitter can come by and be present while you're home so that the cat sees her as acceptable and has a chance to get used to her, that can go a long way toward making her feel more at ease.