This is not a Family Law/Child Custody question. You need to ask a criminal lawyer this question.
For educational purposes only. No attorney-client relationship was created by this communication.
In Texas, people aged 17 or older are considered adults for purposes of criminal charges. So in this case, the proper procedure would be to arrest the defendant, book them into the county jail, and bring them before a magistrate. There is no need to contact the parents.
I would suggest getting your niece a decent criminal lawyer to get her the best outcome possible.