That is heartbreaking! I'm so sorry. This is the downside to being a breeder. Sometimes things don't go right and you have to make hard decisions. I can relate because I just spayed two bitches because of a Bloat issue.
We don't know exactly how Addisons is passed down. It seems to be caused by more that just a simple recessive gene like Von Willebrands, but it is clearly genetic because you can watch it hop down the generations in certain pedigrees. Good article on the Tivin site:
http://www.tivinpoodles.com/articles.htm
As a breeder I would personally not breed a Poodle whose full littermate was diagnosed with Addisons. As a buyer I would also not purchase a puppy by or out of a Poodle whose full littermate had Addisons. If you are serious about being a breeder, perhaps the best course of action would be to spay you girl and look for a new breeding prospect out of a different line.
Here are some web sites devoted to Addison's in dogs:
http://www.addisondogs.com/
http://k9addisons.com/