Does a divorce take place if the wife returns to her husband, after her brother had issued an oath of divorce, by affirming that "he would not return his sister to her husband"?

If you swore not to let your sister return to her husband, and you did not have a specific intention, then it appears that you do not break your oath if your sister returns without your consent. This is because the wording implies that your oath is specific to your action, not merely to her return. When an oath is not accompanied by a specific intention, it is interpreted according to the plain meaning of the words.