A man saying to his wife, "O my mother"

Question:
Honorable Shaykh, this questioner says: It is one of our customs for a man to say to his wife's mother, "O my mother." He dislikes this. He dislikes it because it resembles... It is one of our customs for a man to say to his wife's mother, "O my mother." Is this permissible?

Answer:
This is not disliked in the sense of being merely discouraged (makruh tanzi); rather, it is disliked in the sense of being forbidden (makruh tahrim). That is, one should not say to his wife, "O my mother" or "O my sister," because this resembles zihar. Nor should he say to her mother, "O my mother," because if she were his mother, her daughter would be forbidden to him. Therefore, this is disliked.