What is the way to deal with a wife's sister who interferes in family relationships, spreads rumors, mistreats children, and is characterized by a scowl and a lack of benefit?

Errors made by the husband's sister, such as interfering in the relationship between the spouses and tale-bearing, should be dealt with gently and courteously, as Allah Almighty says: "And speak to people good [words]" (Quran 2:83) and "And tell My servants to say that which is best" (Quran 17:53). The Prophet, peace and blessings be upon him, also said: "Whoever believes in Allah and the Last Day, let him speak good or remain silent."