Here's a perspective: when i was 18, I went with a family on their vacation as a babysitter. I babysat for them all the time already, and this was a family that I knew very well, they were friends of my family before they ever had kids, and I loved the mom, dad, and the kids, of course! They paid for my food and lodging, and I think my parents paid for my airline ticket and spending money. I think I had my own room when one was available (we stayed in a hotel for a week and then also in a rental vacation home for a week). At the hotel, I think I slept in a room with the kids while the parents had their own room (there were cots available, but we just used the beds with me and the little girl in one and the little boy in the other and called it our own sleepover). I do remember having a little free time one night where I went to dinner with some other kids my age that we had met at the resort, and I also spent an afternoon or two shopping on my own. But I spent the majority of my time with the family, and that was great with me. They included me in all activities (scuba diving, sailing, all meals, a day at an amusement park) and covered the costs, even the scuba resort course - which was amazing!
I had the time of my life. My family wasn't the island vacation type, so I was in heaven. I had a blast with the kids, got a fabulous tan without trying, and learned a lot about travel without my parents around to take care of things. I never felt like "hired help" or anything, but I definitely was in charge of the children a lot of the time. I was grateful for the experience and had such an amazing time.
Update: I didn't get paid anything other than having all my expenses covered except airfare. And I don't remember worrying about hours...they were laid back about it and so was I. In fact, I think the time that I was on my own, or "off duty," was their suggestion, not mine. Basically, they were a family that I was familiar with and comfortable with, so it didn't feel like a job to me. If you don't have a similar relationship with this sitter, or if you all or she is not as laid back about it, it couldn't hurt to lay out your expectations as to how many hours per day she is on duty, how many evenings she is on duty, what her free hours would be, etc. Whatever works for y'all.
My overall thought is that if it's someone who is already familiar with your kids and your family and is a comfortable fit with your family, it can be a great way to have some extra help while you travel.