What is the ruling on a wife spying on her husband's nieces by placing a voice recorder for them in her home, and what is incumbent upon the husband to do regarding this?

Spying is fundamentally prohibited, but it is permissible in some cases to prevent evil or avert harm. What the wife did by spying on her husband's nieces is forbidden, and she must repent. The questioner is advised to apologize to his brother and his daughters and to maintain ties with his brother, as maintaining kinship ties is obligatory.