I agree with Evildishrag (I love that name
). I think that your father-in-law is the perfect person to deliver your letter to your bmother. He has been through reunion and understands what the emotions are. Plus, he knows you and can talk to your bmom about you, if she is able to do that at the time he delivers the letter.
Good luck and let us know how it goes! I've got my fingers crossed for you!