Is the polite expression of her opinion by the questioner considered a forbidden retort to her mother, who discriminates in her treatment of her children and disparages her, especially since her father does not do the same?

The mother's behavior mentioned is surprising, as the norm for her is increased tenderness and compassion, so find excuses for her. It is recommended for parents to treat their children equally in everything, and favoritism is disliked, especially in giving, and is not permissible except out of necessity. The mother can be discussed with politely and respectfully. The daughter should be patient and pray for her. It is permissible for a woman to seek a righteous man and marry him, and that might help her avoid problems with her mother.