What is my paternal aunt's share of the two feddans, especially since her grandchildren are demanding half the area, given that the two feddans are registered in the names of my father and my aunt without specifying the share of each?

As long as the official documents state that both your father and your paternal aunt share ownership of this land, and the share of each is not specified, the default is that it is equally divided between them, with one feddan for each. This is if the value of each feddan is equal to the other. Otherwise, the land should be divided according to its value. The word "share" implies equality, and the word "between" implies equal division between those mentioned, and absolute attribution to both implies equality between them. Such matters should be presented to the judiciary, as it is more capable of hearing from all parties and ascertaining the truth of the matter.