Charging decisions by prosecutors can take time, sometimes many months.
Please note that this answer does not constitute legal advice. It is merely intended to provide general information to aid the poster in finding answers to the problem posed. This answer does not create an attorney-client relationship. In most cases, it is best to contact an attorney directly to find answers to your problems.
The girl is probably having an evaluation done with licensed therapists to make sure that this is a valid complaint. Also ,there is a possibility that the girl is being examined by a nurse practitioner to see if there was possible penetration and other signs of sexual abuse.. A lot of times the child wont reveal details until after several therapy sessions which could take months. When and if the district attorney and the sex abuse team decide their is enough evidence to prosecute ,the man will be arrested . Usually the first action in a case like this is the state files a dependent/neglect petition against the stepfather in juvenile court which is not a criminal proceeding but a mechanism to keep the man away from the child until the investigation is complete.
In Tennessee criminal charges are initiated either by Affidavit of Complaint or Presentment.
Affidavits of Complaint are sworn statements made by an officer (or private citizen) which are presented to a neutral and detached magistrate (usually a judicial commissioner). Once the magistrate reads/hears the affidavit, they may decide that there is probable cause the Defendant committed the offense. If so, they will issue an arrest warrant (or summons for less serious offenses).
The other method involves a District Attorney presenting testimony from witnesses before a Grand Jury (12 people who are selected to hear and weigh evidence in these closed proceedings). Once the Grand Jury hears testimony they will determine whether or not there is probable cause that the Defendant committed the offense. If so, they will issue a presentment. A Judge will then issue a capias (arrest warrant) for the Defendant.
As explained above, these cases take time to build. Be patient so that the case can be investigated properly.