Trusted adults do not enjoy the protection of the seal of confession–well, unless of course they actually are confessors–and it isn’t out of place for them to say so as soon as it becomes clear that they’re going to be used as a confidante. It is also not out of place for a sitter to ask the parents, in a theoretical way, what they expect of someone who is in their employment and placed in authority over a teen. Do the parents expect that the adult will pass on information or do the parents expect that their child need adults to talk to, and that the seriousness of things they come to be aware of has to be fairly high before the parents are told? Is the teen even under the authority of the adult, or is the sitter simply making sure that the house is not used by the teen for inappropriate entertaining? Or? This is also the time for the sitter to draw her own boundaries and say what she will and won’t do for the daughter or for the parents, whether she’s paid or not, because her own integrity and her own boundaries have to come first.
It is OK for the OP to tell the teen, “Listen, I’m not just any friend of the family. Your parents pay me to keep an eye on you. I need to talk to them about what they expect of me, and then I can let you know what secrets I can and cannot keep, both for your parents’ sake and because of my own sense of what my duties towards you are.” That is the only situation that is fair to everyone involved, including the one caught in the middle. It could be, however, that the parents will want this person to simply listen and reply to their daughter’s confidences directly to the daughter, and not to be their investigator. Still, just by virtue of the daughter’s age, it is time for everyone to be on the same page, to lay the principles down.