Yes. You are allowed to borrow a car from your relatives, provided that you also manage to get the "student driver' sign put up on top of your car while doing the test. If you don't have one for the top of the car, you can instead place one sign at the rear windshield. Your relatives also have to realise that if anything were to happen to that car, they are solely responsible for the accident because they are the owner of the car even though they are not driving the car. In case of any accident, their insurance premium will definitely increase.
It is a must to have someone who holds at least 4 years of driving experience sit in the front seat beside you while you get to the drive test exam centre. If you fail the drive test exam you will need to have someone with 4 years experience sit beside you in the front passenger seat, to allow you to drive the car back. You do not need a separate insurance for yourself to do the driving test. The insurance company won't give you one. The insurance will fall on the owner of the vehicle in which you are doing the drive test.