I am trying to get car insurance, but the car I drive is someone else's?

I drive my sister's car because it is currently being paid for, leased, i think it is. I do not know anything about car insurance and I recently got into an accident. I must get insurance now because I have court on the 29th! I must show proof of insurance and pay other fees. I need to find an insurance company that is cheap and can do this. Thanks.