Does a wife's divorce take effect in absentia if the husband made her divorce conditional upon his brother divorcing his wife, and the divorce between the brother and his wife actually occurred?

For a divorce to occur, it is not a condition that the wife be confronted with it. As long as the husband conditioned his wife's divorce upon his brother divorcing his wife, and the brother did divorce her, then the divorce of the wife has occurred.

If this pronouncement of divorce does not complete the three pronouncements, he may take her back before her waiting period (iddah) expires.

This is the view of the majority of scholars, while some scholars (such as Sheikh al-Islam Ibn Taymiyyah) hold that a conditional divorce intended for emphasis or prevention, and not for the purpose of enacting divorce upon the fulfillment of the condition, is treated as an oath. Thus, if the husband breaks his oath, his divorce does not occur, but he is liable for an expiation for an oath.