How to deny politely if someone's asking for your car for a day?

My Elder cousin brother is visiting my family tommorow and i am definetely sure he will ask for the car for a day.

How should i deny or what valid excuse i should make to deny him taking my car?

PS: a year ago when i was not home. He asked the car for a day and he will return it in the evening. my mom gave the key he is my Mausi ka ladka BTW. My dad denied but my mom eventually gave it to him.

So eventually evening happened and he didn't come. My Dad asked him where he is and he told him that he is on the way.

He came at night 12AM at night to give the car to us and my Dad was full on anger mode. He came with his friends to give the and car was not even clean and there was dust. My Dad didn't talked to him took the keys and came home. The same night dad called me while i was sleeping and started scolding me "how can you give your car to someone else like this" and i didn't even knew that the car was taken by him.

Edit: matter solved, more details check this: https://www.reddit.com/r/CarsIndia/s/1k2wVg0XiP