How to file case against a person if she is questioning my wife character

Issue legal notice for compensation of defamation (Civil) or ledge a criminal complaint under section 499 / 500 IPC.

It is better to consult and engage a local prudent lawyer for analyses of facts, professional advise and necessary proceeding.

Because of their wicket character, ignore those persons, and do not complicate the issue, as it is not possible to keep everyone shut their mouth, and by proceeding further, you will be giving more publicity.  Let elders and family friends advise her not to resort to such character assassination as it will further destroy relations between their own brothers.  Please do not think that you are being discouraged but these words/actions are like fire and destroy both parties.

How she is doing the defamation? In public with other people present? Or is it online?  In any case can you get witness to support your allegation?

If the answer is No it's difficult. Since she is closely related, better involve other family members, elders and see that she is informed/counselled that what she is doing is bad. It may have impact.  Of course if you take  legal action it's possible that it may silence her atleast out of fear of being punished by Court.
 

Hello Sir,

She is not doing in public she is sending message to my brother and brother is ready to witness as all those messages is stored in his phone 

Can my wife file complain to women's cell 

The facts posted suggest the matter too trivial to institute and pursue; after all, it is a intra-communication between the brother ad his wife!