Is it forbidden for a man to say to his wife: "My sister"?

For a man to say to his wife, "O my sister," is not forbidden (haram (definition)), because she is his sister in Islam or in humanity. However, it is not advisable due to prohibitions against such an expression. The Prophet, peace and blessings be upon him, forbade a man who said to his wife, "O my sister." The one who says this has been considered foolish, meaning that his wife does not become forbidden to him, but rather he is prohibited from using this form of address.