Was Sigrid a Robin in the end? Absolutely not, he never went through that stage even though he would have liked to. Instead he saw his adoptive siblings. After what happened with Jason and the Joker, Bruce sent her somewhere else for a long time. The result? She took the same path as Bruce, she trained in different disciplines but even so, in the eyes of her father, she could not even take on the mantle of Robin or Batgirl, so she created a new personality for herself.

Even if she was accepted, in the end she still feels in the shadows of others, maybe that's why she chose to be a ghost.