However, people got along without electricity and cars and TV and central heating and cooling for thousands of years before today. Without the sun and all the things that it provides, we would be non-existent. Actually, your sister is one smart person. She knows that leaving lights on when you don't need them is bogus and a total waste. It uses electricity, and then power companies have to build extra power plants to produce that extra electricity.
Wasting energy wastes money.
Just one 100 watt incandescent light bulb burning for an hour uses a penny's worth of electricity. Now, a penny may not seem like much until you multiply it by all the lights around the house and all the hours that you forget to turn them off. Even leaving the TV on when nobody's watching can waste up to 6 cents an hour. Add all that up each month and it's enough to buy yourself something you've been bugging your parents for or maybe a nice present for your way-cool sister, who you should listen to more often. All matter is made up of atoms, and all atoms have electrons in them.