Police not supporting

Hi, We filed FIR(498A 3-4 DPA) against my brother-in-law for torturing mentally and physically my sister from last 13 years. They have 2 children and now she is living separately along with her kids(13 yrs daughter and 11 years son). We struggled so much to get FIR filed. My sister went almost 10 times to Police station and waited from morning till night and police will call him to come and finally he will not come. Police will say to my sister come on next day. He came just 4 times overall. Police are not ready to file FIR and every time they were saying some or other reason. Finally, since we are strong to file FIR, they filed at a stage where they left with no other option. From our experience in police station, we can clearly see that he managed police there and they are indirectly supporting him. Now it is time for charge sheet. From neighbors we came to know that when police went for investigation to the flat they just asked watchmen and went to my in-law and spent 2-3 hours chatting with him. By seeing all these, we are suspecting that they will dilute the charge sheet as well. As i understand depending on the strongness of charge sheet, case will run in court. Since my brother-in-law has influence and know many people he is managing at every stage. Can you pls experts help us how we can take right legal help?