Frugal and Energy Efficient Living

Let's face it, even if you weren't there, she'd be using most or even all of the energy and water by herself. So instead of heating one residence all winter at 70 or so and a second at 55, you're only heating one at 70ish. (At a guess)

If you were elsewhere, two lawns would be mowed instead of one (unless you were in an apartment).

Etc.

People crowding together reduces resource use, although as you note, it could be reduced even more if everyone in the crowd could be persuaded to go more frugal.

As far as water goes, maybe you could offset with more efficient appliances? We switched from an older washer when it was getting wonky to a newer one with something called 'precise fill', and our water bills dropped by about 15-20% a month instantly.