RTI is very useful tool if use correctly to obtain lots of information from govt and private organization which can help in your case. For example, you can file RTI in police to obtain copies of complaints, written statements, attendance sheet, and etc.
Here please note some of the police personal may told you that you will not get complaint copy. If they do say that file complaint in DG of police office immediately. it is your constitutional right to get complaint copy.
RTI can also help you to obtain income tax return of wife, father in law, and even brother in law.
RTI can be used to obtain wife working details from EPFO, and from govt organizations.