If you are going to have people “pop in” to check up unexpectedly on your babysitter, please by all means inform her that some relatives will be coming over from time to time to “visit the baby,” and you are letting her know in advance that it is “OK” for her to let them in even if they were not expected (show pictures of the people to her so she will recognize them.
Otherwise, if I were the babysitter and some lady claimed to be the aunt or a grandparent wanting to visit the baby, I would not open the door to them without first speaking to the parents. Any babysitter who DID let a stranger in would be looking for a new job,as far as I am concerned.