Does showing kindness to a relative who used to harm one's mother contradict honoring the deceased mother, and what is the reward for such kindness if it does not contradict honoring the mother?

Your kindness to a relative who has harmed your mother does not contradict your dutifulness to her; rather, it is the better approach, as Allah Almighty says: "And not equal are the good deed and the bad. Repel [evil] with that which is better; then suddenly, he to whom between you and him was enmity [will become] as though he was a devoted friend. But none is granted it except those who are patient, and none is granted it except one having a great portion [of good]." This kindness carries a great reward because it encompasses striving against the self and Satan, and because it is charity and maintaining ties of kinship.