What is the ruling of Islamic law regarding my mother's statement: "I will not forgive so-and-so, even if I am told, 'You will enter Paradise if you do'"?

The mother means that she will not forgive her daughter, no matter how great the reward for it may be, not that she does not want to enter Paradise. It is better for the mother to forgive seeking Allah's reward. She has the right not to forgive and to seek retribution for her grievance on the Day of Judgment, even if she is forsaking what is better. She should be advised to forgive and be encouraged by its immense reward.