What is the ruling regarding a woman who found a love letter from her husband to her sister, the text of which reads: "Yalla, till success in the baccalaureate, my heart"?

A husband addressing his wife's sister with terms like "my heart" and similar expressions is inappropriate. The interaction between them must be confined within the bounds of decorum and Islamic law, without soft speech, to prevent the cause of temptation. Both of them should be advised of this, and the man must adhere to the Islamic etiquette when speaking with a non-mahram woman. If any suspicion arises, efforts must be made to advise them and to prevent their meeting or encountering each other by all legitimate means.