Something that's out of the question is not going to happen — there's not any reason to even discuss it. You could tell your younger sister that her borrowing your car is completely out of the question.
Using the phrase out of the question is a good way to put an end to a conversation. When something's described this way, it's extremely unlikely to happen. Scuba diving without an oxygen tank is out of the question, and driving across the country on one tank of gas is out of the question — these things are impossible. If your roommate says adopting a Great Dane is out of the question, it means she's not even willing to consider the possibility.