My father-in-law was murdered three months ago. I reported the incident at the Banma Itari police station, but the district police station refused to file an FIR. How can I register a formal complaint with the district police station?

You have the right to register a formal complaint with the District Police Station. You can approach the Superintendent of Police (SP) or Deputy Superintendent of Police (DSP) directly and explain the situation. They are obligated to investigate your complaint and register an FIR if the circumstances warrant it.