Is it permissible to warn against a wife who backbites and slanders her in-laws with lies?

It is not permissible to inform a woman about her daughter-in-law's backbiting, as it causes discord between them and constitutes forbidden tale-bearing (namima). The duty is to advise the daughter-in-law and forbid her from backbiting. If she does not respond, then one who can prevent her should be informed. It is also obligatory to advise those who listen to her to denounce the evil and to leave the gathering if she does not stop backbiting.