Does saying to one's wife, "You are unlawful to me in this world and the Hereafter," after her brother swore that she would be divorced, constitute Dhihar, and what are its legal implications?

Prohibiting one's wife varies in ruling according to the husband's intention. If he intended divorce, then it is a divorce. If he intended ẓihār (a pre-Islamic form of divorce by comparing one's wife to an unmarriageable female relative), then it is ẓihār. If he intended an oath, or intended nothing at all, then it is an oath. Its expiation is feeding ten poor persons or clothing them, or fasting three days if he cannot find (the means to feed or clothe).