Helen Stoner's twin sister Julia had died almost exactly two years before Helen came to consult Sherlock Holmes. Helen explained to him that a thorough investigation of her sister's death had been conducted and there was no finding of foul play. Helen would have had no reason to go to Sherlock Holmes at that time. But recently she has been forced to move into her sister's old bedroom and for the first time has heard the strange low whistling sound her sister described to her shortly before her death.
Two days ago some repairs were started in the west wing of the building, and my bedroom wall has been pierced, so that I have had to move into the chamber in which my sister died, and to sleep in the very bed in which she slept. Imagine, then, my thrill of terror when last night, as I lay awake, thinking over her terrible fate, I suddenly heard in the silence of the night the low whistle which had been the herald of her own death.
The very next morning Helen goes to London to consult Sherlock Holmes because she senses that her own life is in danger. The story is somewhat unusual in its structure, because Holmes has to figure out how Julia was killed in order to protect Helen. If Dr. Roylott had not been killed by his own poisonous snake it would have been difficult to prove he was guilty of any crime. It would be hard to prove that Roylott actually sent the snake through the ventilator to kill Helen. He could claim that the snake had escaped from him. And after two years it would be impossible to prove that Roylott had actually murdered Julia Stoner with the same snake. At worst, it could only be hypothesized that the snake had gotten loose and crawled into Julia's bedroom on its own initiative, seeking to escape from captivity. When Roylott is killed by his own trained snake, it ends the story neatly. Helen Stoner will inherit all of her mother's capital and be happily married. From what Watson says at the beginning and again at the end of the story, it appears that Dr. Roylott was never accused of murdering Julia Stoner or of attempting to murder her sister Helen. Watson asserts that he is telling the real truth about these occurrences for the very first time.
b,coz helen had not experienced what her sister experienced
so, when she was about to marry all the incidents happened to her.