Is what the sister did by mentioning the actions of her brother's wife unnecessarily, or what the second sister did by informing the brother's wife of what her sister said about her, considered backbiting (gheebah) or slander (nameemah)?

If the problem concerns the father and it is necessary to mention what the brother's wife did to fix it, this is not considered prohibited backbiting if the intention is reform, hoping for change, and reducing harm, and if it is hoped that the sister can change the situation because the brother's wife responds to her. However, if it is merely to vent anger, it is not permissible unless the woman is causing harm. The sister erred by engaging in both backbiting and tale-bearing, because she conveyed a statement that led to enmity. The sister is considered among the tale-bearers unless there is a benefit or advantage in it. She must repent and strive to mend what she has corrupted. A tale-bearer will not enter Paradise. The best speech is that which reconciles people; for the reconciler is not considered a liar if he intends good.