According to the TN Department of Safety a conviction for driving without being licensed (or having a permit or certificate for driving) is a class B misdemeanor. The penalty is a fine up to $500 and/or 6 months in jail.

Instead your sister may have been cited under TN Code Section 55-50-351 (License to be carried and exhibited on demand - Arrest and penalty for violations). Under section (a) it states that every licensee shall have such licensee's license in immediate possession at all times when operating a motor vehicle and shall display it upon demand of any officer or agent of the department or any police officer of the state, county or municipality, except that where the licensee has previously deposited the license with the officer or court demanding bail, and has received a receipt from the officer or the court, the same is to serve as a substitute for the license until the specified date for court appearance of licensee or the license is otherwise returned to the licensee by the officer or court accepting the same for deposit. Any peace officer, field deputy, or inspector of the department, or any other law enforcement officer of this state or municipality thereof, has the right to demand the exhibition of the license of any operator of a motor-driven cycle as described in § 55-8-101, and effect the arrest of any person so found to be in violation of this.

Subsection (b) goes on to say that a violation of this section is a Class C misdemeanor.

In TN a Class C misdemeanor usually has a penalty of jail time not greater than thirty (30) days or a fine not to exceed fifty dollars ($50.00), or both.

Your sister should try contacting the court listed on her ticket to find out if she can bring her driver's license or some other proof of having her licensing status in to the court to have her citation for driving without a license or failure to display her license on demand dismissed.