Most reputable car insurance companies only offer policies in six or twelve-month terms. In some instances, you may be able to get a policy for a month, but that’s rare.
If you aren’t interested in a non-owner policy, speak with your sister. Her car insurance will likely cover permissive use of her vehicle. That means, as long as you are borrowing her car with permission, her insurance should cover you. She can call her car insurance company to confirm.