What is the ruling on family mixing that does not observe the Sharia guidelines, what is the sin resulting from it, and is speaking about it considered backbiting?

It is not permissible for the wife of the husband's brother to display her adornment in front of her husband's brother. Nor is it permissible for your husband to sit in a gathering where his brother's wife displays her adornment. They must be advised gently and kindly. If it is not possible to separate the gatherings, then the brother's wife must adhere to complete modesty, or the husband must avoid the women's gathering. A wife speaking to her husband about his brother's wife to rectify a wrong is not backbiting (gheebah) that is forbidden.