Does a void contract have to be declared to be void by approaching a court?

The agreement is not voi neither it was breached by either party.

In fact the agreement was acted upon by the parties to the agreement hence no one can claim that it is a void agreement or it was breached.

Of course the agreement conditions cannot be enforced in court of law because it was not executed by a registered deed, but it can be used as a substantial documentary evidence to prove the intention and establish the facts.

You don't have to budge to any pressure, you can simply reject their claim and challenge it if they decide to approach court.

In fact your sister in law is a third party to this agreement or the properties that were divided on the basis of this mutual consent agreement. Hence you don't have to even entertain her.

Thank you sir.  you r right. it is my sister in law who is problem creator but she controls my brother.  My local advocate says my brother should have asked to void it in three years but now she cannot. is it true? Can he go to Court now and get order that it is void  - I do not know legal term for that? 

She has no locus standi to question validity of the agreement between brothers over partition of ancestral properties which may be reduced in writing or even oral partition is valid, legal and enforcable. 

Besides this, the agreement is dating back to 2016 which has attained finalty, complete and binding on the parties to the agreement.

I fully agree and appreciate the opinion and advise of learned senior expert Mr. T Kalaiselvan.