The one thing that the families could do (I don’t know if they already have) is to ask the CPS to review their decision. If they do this, it is reviewed by a different CPS lawyer to the one who did the initial review and made the decision.
But as far as I am aware, the defence team do not get to present the defence case to the CPS before they make the decision to prosecute - the ‘clever lawyering’ that @busymomtoone mentions doesn’t swing into action until after a decision to prosecute has been made.
The CPS only examines and considers the prosecution side’s evidence - anything that would prove that the person DID do the crime. If they consider that evidence does prove that the person committed a crime, they then consider whether prosecution is in the public interest - but they only go on to that step if they say a crime has been committed.
So they look at all the most damning evidence in the case, and none of the defence’s evidence - you could say it is a very one-sided process because it only considers the evidence that should prove that the accused did the crime and the defence doesn’t get a say.
The lawyers at the CPS have looked at the evidence and found it shows that no crime has been committed - the woman had no clue that she was going to have a fit at the wheel of her car, she hadn’t been negligent about her health, she didn’t deliberately drive when she knew she was impaired.