If my mother co-signed for a car for my sister but my sister did not insure the car and got into a fender bender, could my mother be held responsible?

If your mother's name is on the title to the vehicle, which is likely if she is a co-signor on the loan, yes your mother can be held legally responsible for any accident or damage which may be caused by that vehicle.  This is unfortunate that your sister would put your mother in ths position.  Hopefully the damage is not significant and hopefully your sister will do the right thing and take responsibilty for the accident and the fact that she failed to insure the car and cover this herself.