In my opinion, unfortunately if her owner already is negative about her I doubt she'd change her mind. It sounds like she expected a perfect dog and doesn't want to work with her to help her be the good dog that she could be.
If I were you, I'd start looking for a new home for her. At least she's not in danger and is not being abused or neglected so you can take you time in finding her a great home ... humm.. Maybe she wants to live with her sister!

I'm so sorry it didn't work out. She is such a beautiful little girl.

I wouldn't stop trying to get her owner to keep her, maybe your persistence will pay off in the end!