The longer you carry on doing it, the worse the problem will get. If I were you, I would wash your own stuff, and perhaps the stuff of this person who cooks for you. Fair's fair. Clean the sink/surfaces if you want to. But this other girl? Gather her stuff together and stick it in a dirty pile in the corner, preferably on the floor, with a note n top saying something like, "Sorry, I was looking for the sink" and leave it there. If you don't wash her stuff, she will have to. It's as simple as that.