All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If your father had taken possession of that piece of land when his mother was alive and she gifted it to him at a time other than the time of her death sickness, then this was a valid gift according to the view of some jurists and an invalid gift according to the opinion of other jurists.
The view we adopt here in Islamweb is that this gift is unjust and Islamicly invalid, and it should not be taken into account and this land does not become the property of your father because his mother was unjust in that gift as she gifted her son and did not gift her other two daughters. The Prophet called such a gift an unjust gift and refused to witness it as he said to the companion who gifted such a gift: “Fear Allaah and be just between your children (when gifting them)…" Then that companion stopped that unjust gift.
Similarly, your grandfather gifting his two daughters without gifting his son is an invalid gift. It might be that what led him to do so –in our opinion –is that your father refused to give back the piece of land which was gifted to him by his mother.
Therefore, it would be more correct for your father to give back that piece of land and that your aunts should give back that house, and then the piece of land should be divided among the heirs of your mother, and afterwards the house which is possessed by your aunts should be divided on the heirs of your father, as well as the house and the money which your grandfather had left; all this should be divided among the heirs according to the Islamic Law. By doing so, you would have corrected the mistake which your grandfather and grandmother had done. You should know that the inevitable result of not abiding by the Law of Allaah is dissension, disagreement and disunion, as Allaah Says (what means): {But if they had done what they were instructed, it would have been better for them and a firmer position [for them in faith]. And then We would have given them from Us a great reward. And We would have guided them to a straight path.}[Quran 4:66-68]
Allaah Knows best.