Is the gift that a father bestowed upon his two sons, to the exclusion of his daughters, during his lifetime, by way of recompense and reward for their service and care, Islamically permissible, or must the sons give a share of this land to their sisters?

The most cautious and absolving for one’s conscience is to return the unjust gift (favoring some children without a legitimate Sharia reason) after the death of the giver. This is the choice of Sheikh al-Islam Ibn Taymiyyah, because the Prophet, peace and blessings be upon him, commanded justice among children. It is not permissible for the one who was favored to take the excess; rather, he should share it with his siblings. Likewise, filial piety towards the father or working with him for free is not a justification for favoritism.