Get a third key?
 

I want to get a third spare key for my sister in case she needs to drive it. I want to know how much usually the spare key cost and how long does it take to order one from the dealership.

What year/model car is it? Most GM dealerships will have keys in stock. If it is one that has a resistor chip in it, they usually charge around $45 here for the ignition key, and cutting it. You can also get spare keys cut at most locksmith shops.