The best way to be certain that you are properly covered in case of an accident in your relative's car is to have them check with their insurance agent to make sure their insurance coverages would be extended to you when they loan the car to you for the day.
Typically insurance coverages will be extended to friends and relatives living outside of a household without them being placed on the policy as long as they are given permission by the owner to drive the vehicle. There are some policies that have certain exclusions though that keep the policy coverages from being extended to other drivers, even if given consent to drive the vehicle.
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