Does the utterance "You are like my sister" to one's wife, said jokingly while fasting, with the intention of making sexual intercourse impermissible during the fast, and not with the intention of ẓihār, constitute ẓihār?

If your likening your wife to your sister was not an act of zihar (injurious assimilation), but rather a narration of the reality of her being forbidden to you due to fasting, then this is not zihar. However, it is disliked in Islamic law, due to the statement of Ibn Qudamah: "It is disliked for a man to call his wife by the name of someone who is unlawful for him, such as his mother, or his sister, or his daughter." This is because it resembles the wording of zihar, but it does not establish prohibition or the ruling of zihar.