Reasonable to expect her to clean up after herself. I sure wouldn't want to return from a vacation, tired and possibly jet-lagged, to find I couldn't use my kitchen until I'd washed up a load of someone else's dishes.
Given her age and that she is family, and that she has never lived away from parents, I think it's reasonable to lay down a list of rules.
I had a house/catsitter while I was away for a month overseas, and when I came back, everything was pretty much fine, the only thing was he hadn't put on fresh sheets (the only bed was mine, no spare room). It was a little annoying after coming home from a month away and a 10-hour flight, but then again, we hadn't talked about it in advance, he'd done a good job looking after my animals, nothing was broken or missing, and the place was reasonably clean. He'd saved me a bundle in pet services. So while it was a drag to come home to a bed that I had to change, it wasn't the end of the world. If I were in his shoes, changing the sheets would be a given, but I guess not to everyone. So if I ever need a housesitter again, I might just say, "Oh, and if you don't mind, before you leave can you put on fresh sheets? I'll be coming in from a long haul flight."
Again, given Jenny's age and maturity level, I don't think it's unreasonable to say (in nicer words), "please don't make us come home and find our house in a tip."
As for the food, I think it's fine to separate what she can and cannot help herself to. I've house-sat before and generally I don't touch their food, save for a few basics like a bit of oil for cooking, etc.