The Will shall prevail over the succession law concerning the parties, provided the Will is a valid one and that can be confirmed at the stage of probation in the court of law.
If the father made a Will in favour of the son and did not make the daughter beneficiary of any of his property, even in such case the son or the executor named of the Will has to first get the Will probated in the court, where the notice to all the legal heirs of the deceased will be send to defend the case.
The legal heirs of the deceased have right to challenge the Will at that stage in the court. The court shall go through the objections raised and decide the genuineness of the Will, only after this the letter of administration issued in favour of the beneficiaries for necessary changes to be made in the revenue record or the bank accounts etc to ultimately get the property of the deceased.
The matter relating to the nature of the property in the hand of the deceased, if that was his self acquired or ancestral will also be decided in the court proceedings, as such in this case this being ancestral in nature is doubtful as this was not three generation old property in the hand of the deceased person.
In case the Will is found to be void, than both son and daughter get equal share in the property if they both are the only legal heirs of the deceased.
EDIT: The fact about 100years old family business for existence for more than three generations definately goes in to show it being ancestral in nature, however whether all the properties of the deceased in question are ancestral in nature or not has to be decided by the court on the basis of evidence regarding this, a Will can be made with regard to self acquired property only not ancestral property by the holder of the property,this fact will be adjudicated in this case too.
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