We suspect the oldest girl is Belle.
She has always been somewhat timid and fragile. She has been a mother and a nurse / aunt to the litters. I have noticed she is starting to show some signs of aging and I am a little concerned with her well being, especially when it comes to her navigating the habitrail and it's tubes. And I have seen at least one younger female bullying her.
Belle and Pudding are the Matriarchs of the brood and they have spent most of their lives together. When they were put in with the females of their collective litters they seemed to drift apart. But, I have seen them snuggling together when they are not overwhelmed by their children.