Hello Friend,
If your grand father had acquired the property out of his earning and that was his absolute property, he has the right to dispose it of in a manner he likes. Then you have no rights if he has willed it to your brother. I think obtaining probate from a Court is not necessary in this case for the will to be effective.
But, as opined by Mr Sarin, the validity of your grand father's will is always open for contest. If you think you can establish before a court of law that the will was only result of tax adjustment arrangement within the family, or your grand father was not maintaining reasonable mental faculties at the time of execution of will, or is not registered one, etc., etc.,, I think you have a good case of partition suit.
Good Luck
A W K Mecci, Bangalore.