Short Answer: Julia dies after being bitten by her step-father's venomous swamp adder.
At the beginning of the story, Helen Stoner visits Sherlock Holmes and Dr. Watson and explains to them that she fears for her life. Helen goes on to mention that she views her step-father, Sir Grimesby Roylott, as a threat to her well-being and explains to Holmes and Watson that her sister, Julia Stoner, died two years ago. Helen says that Julia died shortly after she got engaged and the events surrounding her mysterious death are eerily similar to her current situation. According to Helen, on the night of Julia's death, her sister mentioned that she had heard a mysterious low whistle for the past three nights. Later that night, Helen was awakened by Julia's screams and happened to hear a low whistle whenever she opened her door. Before Julia died in Helen's arms, Julia told her,
"Oh, my God! Helen! It was the band! The speckled band!" (Conan Doyle, 4).
At the end of the story, it is discovered that Julia was bitten by a venomous swamp adder, which Sir Grimesby Roylott trained to travel between his room and Julia's room whenever he blew a low whistle. Eventually, Julia accidentally provoked the venomous snake, which caused it to bite and kill her.
After Helen got engaged, construction started on her home and she was forced to stay in Julia's former room. She also heard the mysterious whistle, which motivated her to seek Holmes's services. It is discovered that her step-father also planned to kill her using the same method, but his plan backfires after Holmes intervenes.