Liability

If you have not already done so , you need to advise your insurance company that your son-in-law used your car without permission and was involved in a hit and run incident causing damages.  You should also file an incident report with your local police department advising them that your son-in-law took your car without permission and was involved in a collision.  If you do all of the above, you will likely not have any personal liability for the incident.  However, your son-in-law will have personal  liability/responsibility for the incident.  Good luck with this matter.