Is the inability to feel mercy for one's mother due to her extreme injustice and her favoritism of one brother over the rest of her children considered haraam (forbidden)?

Parents must not show favoritism in their treatment of children and should be just among them, based on the Prophet's (PBUH) saying: "Fear Allah and be just among your children."

Nevertheless, children are not permitted to respond to parental harshness with harshness. They should remember that Paradise lies beneath the feet of mothers, and that the father is the middle gate of Paradise. As the Prophet (PBUH) said: "The pleasure of the Lord is in the pleasure of the parent, and the displeasure of the Lord is in the displeasure of the parent," and "The father is the middle gate of Paradise, so if you wish, you may neglect that gate or preserve it."

Therefore, one must forgive the mother and supplicate and seek forgiveness for her after her death.