I always assumed she was one of Mrs. Long's nieces. We do know that the nieces were there because Mrs. Bennet mentions them afterward.
I wonder if the nieces, however many there are, are living with Mrs. Long. Perhaps their parents have died. In the beginning of the book, Mrs. Bennet does not think Mrs. Long will introduce her girls to Mr. Bingley because she has nieces of her own; I have to think that if the girls lived elsewhere, Mrs. Long wouldn't have been quite so concerned about finding husbands for them around Longbourne.