If it is the self earned property of the parents of your wife, the parents have every right to do whatever they want with the property. In other words, it will be perfectly their right to give / gift away the entire property to some third party - completely ignoring their own sons and daughters etc. - no body can question their action.
This being so, needless to say, during their life time if they give the property in unequal share between their sons and daughters, no one can question, and no one can legally ask for more share. Similarly, if they leave any will in favour of anybody bequeathing the property in whatever proportion the same cannot be questioned, unless the will itself is challenged.
However, if they do not give any share in their property during their life time, and do not leave any will, then the property will go to their children by way of inheritance in equal share.