My memory of Aunt Sally is as a character from a children's TV show from around 30 years ago called "Worzel Gummidge". The title character was a scarecrow who came to life when adults weren't around. Aunt Sally was a shop mannequin who was the object of his affections.

I don't know where you heard or saw this reference, but it has no particularly special meaning in the U.S. (that I am aware of). I know that there is a game called Aunt Sally played in some British pubs, where sticks are thrown at a doll's head, trying to break a clay pipe in its mouth. From that game, an "Aunt Sally" has come to mean something or someone that is an ' easy target for attack or criticism. But the phrase would mean nothing to an American, and I'm not sure how common it is in Britain.