“Car insurance
policy numbers typically aren’t considered personal data. You wouldn’t want to freely give out the information, but it is safe to give it to your sister’s insurance company. Her car insurance company is asking for your policy number to verify that you do have insurance. Most car insurance carriers require policyholders to list any household members and whether they have a driver’s license and their own car insurance.
If they are going to be excluded on the policy, insurance carriers want to know why they’re being excluded. Having your own car insurance is a reason someone can be excluded.
If you don’t want to give your sister’s car insurance company your policy number, they may be able to find it through the reporting they pull. If they can’t find it, she may be required to add you as a driver on the policy.”