The fridge wars

We usually have two bottles of cold water in our fridge. The rule is that if you empty one, you refill it from the tap. Simple enough you would imagine.

(1) But what about when one person is getting another person a glass of water? Do the rules change then?
(2) And how is this affected if person A was doing so without person B's knowledge or consent?
(3) What about when one person, (let's just, for arguement's sake, call them, oh I don't know, my brother) when filling up their glass, deliberately leaves the tiniest trickle in the bottle? Usually in this case, he knows it's nearly empty and doesn't go near it, while everyone else assumes it's already empty, but won't fill it up cos they know they didn't empty it. Which leads me to
(4) When someone, again my brother for simplicity, knows the bottle is nearly empty, and so switches to drink milk or something else that he normally wouldn't touch with a ten-foot pole.
And my current biggest annoyance:
(5) Bottle A is nearly empty. Everyone switches to drinking from Bottle B. The end result is two bottles with less water in them than rain in the Sahara.

Drink a bottle. Fill it up. It's not that hard.