Do you need to be added to your parent's insurance in order to drive their car?

Insured drivers
If you have a permit and reside with your parents and have a license or permit, you should be listed on their policy even if you do not intend to drive the car. If not, a claim could be denied if you did drive it because the insurance company was not informed of this risk. Their rates will be higher, but not that much. It would be a larger increase if you were a boy.

If you live in the same household as your sister, everything above applies to your situation, too. Otherwise, if you have an unrestricted license and use her car only occasionally, her insurance will cover you unless there is a specific clause in her policy which excludes other drivers.