You can definitely approach the higher authorities in the police department, such as the Superintendent of Police of the district or the Deputy Commissioner of Police or Commissioner of Police, as the case may be. In fact, you should approach them if the police at the police station level is not taking keen interest in the case.
If you suspect that you sister might have been kidnapped / abducted by someone (known or unknown) or that some other offence might have been committed, in that case, police should have registered FIR and investigated the case. In such a situation, if the FIR is not registered, then you may approach the Magistrate court under Section 156(3) of the Criminal Procedure Code to direct the police to investigate the case.
If need be, you may even approach the High Court by filing a writ petition, to seek directions to police to properly conduct investigation / enquiry in the case.
You can also request for transfer of the case to the CID / Crime Branch, etc. This can be done by requesting the senior police officers, or by filing a petition in the High Court.
Dr. Ashok Dhamija is a New Delhi based Supreme Court Advocate and author of law books. Read more about him by clicking here. List of his Forum Replies. List of his other articles. List of his Quora Answers. List of his YouTube Videos.