Here is the Aggravated Assault statute in Illinois.
A Class 4 felony is the lowest level felony in Illinois. it carries a maximum penalty of 1-3 years in prison and/or a fine of up to $25,000. The misdemeanor resisting arrest, is an A misdemeanor with a maximum of a year in jail and/or a fine of up to $2,500.
The aggravated assault would be an A misdemeanor too under most circumstances, unless she used a weapon or caused serious injury. Here, it's being bumped up because the victim is an officer.
She was drunk and has never been in trouble before. Although I can't guarantee anything, of course, in my experience, as long as the police officer didn't really suffer an injury, the prosecutor will reduce this to a regular assault or dismiss it and just go with resisting. I do not expect that this matter will remain a felony for very long.
I also don't believe that jail is going to be a real possibility, unless your niece fights the case and loses. If she wants to dispose of this, I think probation would be very possible. And dependiing upon the facts and circumstances here, she could possibly do better than that.
She should have a lawyer, because if there is any possibliity of resolving this case in some way that wouldn't give her a criminal record it will be because of the efforts of a lawyer. If she can retain one, she ought to have the lawyer with her at her arraignment. If she cannot, she should plead not guilty when she goes before the judge and ask the judge to appoint her a public defender.