Partition in Muslim during his life time is valid or not

Dear Sir

Brief note on our case : Our Grand father has 5 sons , Iam the grand son ( Son of third Brother ) ,Out of these 5 sons , 2 of them are claiming that partition is already done by my grand father & no property is alloted to my father , So now I cant ask my share in grand fathers property.Is it valid in Law - Please clarify