In Arthur Conan Doyle’s “The Adventure of the Speckled Band,” Helen and Julia’s surname is Stoner.
Helen Stoner visits Holmes because she fears for her life after the unexplained death of her twin sister, Julia. Julia had been staying at Stoke Moran, the family home in Surrey, and mentioned a mysterious “speckled band” shortly before dying.
Holmes investigates the bedroom and discovers that the apparent supernatural threat is a deadly snake trained to travel through a ventilator and bell-rope. The villain is Dr Grimesby Roylott, Helen’s stepfather, who wants to prevent her marriage and preserve his financial interests.
Readers sometimes remember Roylott more readily than Stoner because he is the story’s memorable antagonist. Julia and Helen, however, are the sisters whose family name supplies the answer. The story first appeared in The Strand Magazine in 1892.