When I cat-sat my sister's kitties, the vet's office had a note in each of their files with her consent for the cat to be treated and her credit card number, just in case. Nothing ever happened.

The only other thing I told my pet sitter is which key opens which door, feel free to help yourself to the TV and radio, don't bother to water the plants, open the balcony door if it's nice out (but close it when you leave), and there are cans of pop under the kitchen table and ice in the freezer so please have one if you like. She never does but I always offer.