Mark C is right and there is really too little to go on with the limited information given, but I would just make one point. You say "A few years prior to that a piece of property was given to two of the siblings". How many years? If less than 7, it is likely to be included within the Estate for IHT purposes, either at the full or tapered rate depending on how long ago.
Other than that, I assume the issue is that the dissenting sibling feels that the others have had some of their 'share' during your grandfather's lifetime and that the division of the estate should reflect that. That comes down to what the will actually says, what your grandfather intended, whether he was rational when he made that decision etc etc. If I were the 'dissenting' sibling, I would simply have a conversation with the others but other than that it's off to the lawyers for an expensive and uncertain outcome.