Does a husband incur the expiation of ẓihār—fasting for two consecutive months—if he said to his wife, "You are forbidden to me as my mother," after he had already paid an expiation for an oath based on a previous fatwa, and then his wife remembered that he had likened her to his mother?

If you recall saying to your wife: "If you do such and such, you will become like my mother and sister," this is a conditional ẓihār, and its expiation becomes obligatory upon you if your wife carries out the act it was conditioned upon. If you do not recall, but you believe your wife, then you must perform the expiation for ẓihār if your wife carries out the act it was conditioned upon. However, if you are in doubt, you are not obligated to perform the expiation for ẓihār. The expiation for a broken oath which you have already given will suffice, because the ẓihār is doubtful, and you are not required to act upon your wife’s statement if you do not believe her.