So many decisions can factor into a Judge's decision and unless I know them all, I have faith in the Judge.
Circumstances that could have factored in this case: the age of the woman who pled guilty, the circumstances surrounding her being legally drunk and driving (was she blotto or just over the limit and not know it), the fact that she pled, whether or not she had a criminal record for drunk driving or other crimes, her level of remourse, the feelings of the victim's family, restitution paid, whether or not the injured parties contributed in any way to the incident, whether or not she completed an alcohol treatment program and seemed genuinely on the road to rehabilitation.....these are just off the top of my head - there are probably others.
I'm not saying any of these factors (or others I'm not thinking of) were taken into consideration in this case. But if she pled, both the prosecutor and the defense had to be comfortable with what type of punishment would potentionally be levied against her.
I think the sentence is light, but I'm willing to believe that the Court has a better vantage point of the evidence.