I may be more old fashion than this modern world can handle. People in my house are guests. Unless they are here for a prolonged stay, They are treated as guests.
My DMIL, who lived with me for about thirty five years, always took your dishes before you were even finished eating and stacked everything from glasses to pot and pans in a sink of water, to soak. There was no relaxing over a meal when she decided dinner was over. Then every greasy pan piled high in the sink would prevent anyone from getting a glass of water or fill a watering can or anything you do at a sink, unless you emptied it first. Oh, how I hated that. She is the same woman who has a daughter who tells people when they can speak, sit or stand... or fart.
My DH is blissfully free of these kinds of domineering habits.
Now that I am living in a MIL free zone, I can freely live the way I want. And I insist that visitors are treated as such.
This is a long way to say, a glass of water is a little thing. Just be happy she visits.
peace