Is the son obligated to return a quarter of the house that his mother gifted to him through a contract of sale and purchase without consideration, and would he be sinful if he did not do so, given that she regrets it and wants to reclaim it?

A gift of an undivided share of a house is valid according to the majority of scholars. If the gifted item has been taken possession of, the mother may revoke her gift to the questioner's brother, based on the Prophet's (peace and blessings be upon him) saying: "It is not permissible for a man to give a gift and then take it back, except for a father in what he gives to his child." The mother is like the father in this ruling. An exception to the parents' right to revoke a gift is when the child has made an alteration to it that changes its value, or when the child's status has changed by getting married or incurring debt.