Is the grandfather entitled to 10 acres of land that was released from mortgage after his father promised him that, knowing that the grandfather's father had only one son and one daughter?

If the intention behind "redeeming the mortgage" means the grandfather's efforts with the mortgagee to release the mortgage, then there is no harm in that. The grandfather is entitled to the ten feddans as a "ja'l" (payment for work) in exchange for his efforts. The presence of the grandfather's sister does not affect the validity of the transaction or his entitlement to the "ja'l."