I only want to say that your W will trust that you are no longer pursuing her, WHEN and ONLY WHEN she chooses to believe that.
No words are going to convince her of that. While I accept what your brother says about conveying that message to your X, I'll just expand on that by saying / repeating... It will be your continued actions of NOT pursuing your W that will eventually lead to her believing it...
So he is right, I just don't think words will make that happen... time and consistent action will... eventually...