Praise be to Allaah.
If your marriage was contracted first and was done according to the conditions of sharee’ah, then she is your wife, and your brother’s marriage to her is invalid and does not count. He has to repent to Allaah from what he has done, and you and your wife also have to repent to Allaah if you helped him to do that. Let your wife rest assured that your marriage to her is valid so long as all the conditions required in sharee’ah were fulfilled. And Allaah is the guide to the Straight Path.