Is it permissible, in Islamic law, to take back part of a plot of land gifted to a sister, specifically the area where her children resided after they benefited from state housing, while leaving the area where the sister resides?

It is not permissible to retract a gift after the donee has taken possession of it, due to the Prophet's (peace and blessings be upon him) saying: "The one who retracts his gift is like a dog that returns to its vomit." An exception to this is the father, for whom it is permissible to retract his gift to his child. Similarly, it is not permissible for the giver of a gift to buy back what he has given, due to the aforementioned hadith (definition) of Umar (may Allah be pleased with him).