What should I do about telling my wife, "If you go out with your brother, then you are divorced," if I want to retract this condition?

The majority of scholars, including the four Madhhabs, are of the view that a contingent divorce takes effect upon the occurrence of the condition it was contingent upon. This is the preponderant opinion.

So, if you made your wife's divorce contingent on her going out with her brother, and she went out with him in the manner you intended, then the divorce has taken effect according to the majority. If the divorce has taken effect, the oath is dissolved, and its consequences cease. You may revoke it before the expiration of her 'iddah, provided this divorce does not complete the three divorces.

Shaykh al-Islam Ibn Taymiyyah is of the view that an expiation for an oath becomes incumbent upon you if you did not intend divorce but rather intended prevention or threat. The oath or divorce does not take effect if you intended her not to go out for a specific period and then she went out after its expiration, or if the reason for which you swore ceased without any action from you, because the oath is based on the intention of the swearer.

While the majority are of the view that a divorce oath is binding and cannot be retracted, Shaykh al-Islam Ibn Taymiyyah permitted retracting the oath if you intended divorce, not just an oath.