How about something like:
"[her name], I hope you know I love you like a sister. And [your hubby's name] has never stopped loving you. We want you to know that, after much discussion and consideration, we've decided to open up our marriage. In other words, we are now non-monogamous.
I'm telling you this because, if you still feel the same way you did for [hubby], you both have my blessing to resume your relationship. I know he would want that very much, and you should know that I have worked on and gotten past all the issues that made it difficult for me when it was an affair between you two. What we both want now is for it all to be out in the open and honest. In fact, we've even discussed the possibility of asking you to relocate to be nearer to us. That's how serious we are about this.
I know this must come as a shock, but it is real, and [hubby] and I both want you to seriously consider this, and to come to us with any questions you might have."
You could write it too, if you think you would have a hard time speaking to her.