Is it permissible to inform the husband's family of what their other daughter-in-law says about them, and does sin befall the doer or the listener who does not denounce it?

Regarding what the wife says in backbiting about the husband's family, you must do two things:

First: Forbid the wife from backbiting, because it is a reprehensible act and a grave sin. The listener must rebuke the backbiter, or leave the gathering if the backbiter insists on their sin.

Second: Do not convey what that woman says about the husband's family to them, because this is tale-bearing (namima), which is one of the major sins. Al-Dhahabi clarified that anyone who incites discord between two people and conveys between them what harms one of them is a tale-bearer from the party of Satan. It has been authentically narrated from the Messenger of Allah, peace and blessings be upon him, that he said: "A tale-bearer will not enter Paradise." A tale-bearer is one who transmits speech between people in a way that harms one of them or causes resentment in their heart, unless there is a benefit or advantage in doing so.