Who will be the legal heir

To which religion your father belongs? Was he a Government Servant at the time the second marriage (legally or otherwise) had taken place 

This is not a simple problem that can be sought from a public forum and acquisition of property with or without ancestral property's help etc. counts.  Though second wife may not be having shares, her children are equally eligible for proportionate share.  It is also not clear whether your father remarried after the death of first wife or during her life time, with her consent.  It is always better to consult an advocate and no person can give such relinquishment so easily and sign papers.  Further all relinquishment deeds on immovable properties must be registered.  Trust an advocate, arrange family meeting and get a settlement deed within the family to avoid future complications and fights within sibblings. (There were many cases reported in the forum where own brother and sisters are fighting in the street for their rights, you are talking about obtaining signed papers from step children)

Dear Both,

First of all thank you for the immediate update.

To answer you queries.

During 2nd wife arrangement{illegaly} - Father is a Government servant and belongs to HINDU religion. And 1st WIFE is LIVE and with out her knowledge / consent - father had betrayed and married 2nd wife illegaly. Though married 2nd wife,but father stayed with us {1st wife} and staying with us only.

Thanks.

Although the second marriage is invalid and your father can be punished for such act of marrying another lady while first marriage is subsisting, the children born out of such second marriage are equally eligible for the assets of the father, either ancestral or otherwise.Better a proper settlement is arrived at through a registered legal document with the help of a competent and honest Lawyer.

Dear Sir,

Many thanks for clarrifying.

We were thinking all the properties/rights in father name can be transfered to 1st wife offcourse with the father consent. In this case, later whether is it legal or void for 2nd wife/wards claims in later stage ?

Any advice.

Thanks.

As already stated, the children of second wife are equally elitble to share the assets of your father. Therefore, you have to seggregate the assets of your father into two categories ie. self acquired and ancestral. Self acquired properties, he can write a Will and give the assets to whom so ever he wants. Nobody can question.  Of course, when such Will is contested, it will be contested and the onus of proving that the property stated in the Will are self acquired and not ancestral, shall be on the beneficiary of such Will.

In the case of ancestral property, the other children too are quite eligible to share and can successfully contest. 

Thanks for the update sir :)