if the computer's on, and you're away from it and the monitor is turned off, you're talking about a regular lightbulb.
If the computer's on, and you're using it with the monitor on, you're talking about 2 or 3 regular lightbulbs.
So if your sister is willing to start paying every time she turns on the light in her room, then maybe it's time for you to start paying for your computer
But dry one load of clothes and you've used FAR more energy.