Sold car to my sister. Hasn't transferred title and rear-ended someone today. -AZ

Like the header says. We sold her the car, notified DMV it was sold to her and canceled our insurance last week. This morning, she rear-ended a lady and there is damage to lady's bumper. What are we possibly looking at liabilty-wise, because sister hasn't transferred title and, of course doesn't have insurance?

Sister gave the lady the insurance card that was in glove box but we had already removed that vehicle from our policy and notified our insurance of selling it.

Any help or advice is greatly appreciated.