I would have the housesitter come in while you are still there to get the dogs used to them. Show the sitter what you do to feed and water and exercise them while they are watching. Then start having him/her do these things while you are there and able to show the pets that you "approve" of what the sitter is doing. Make sure that you have their favorite treats around to give them when they show good behavior. Have the sitter come and pay attention to them, give them treats, play ball with them if they do that. If you have pet names for them make sure the sitter knows them. A lot will depend on your attitude toward the sitter, try to be relaxed, calm and generally adopting a "what a great, fun thing this is" pose.