The real reason that Helen Stoner is currently sleeping in the room of her deceased sister, although she does not know it, is that her stepfather Dr. Grimesby Roylott wants to kill her the same way he killed her sister Julia two years earlier. Julia's former bedroom is directly next to Roylott's, and there is a ventilator between the two rooms. Roylott killed Julia by sending a poisonous snake through the ventilator. It climbed down the dummy bell-rope and onto the sleeping girl's bed. Roylott knew that the snake (the "speckled band") would not necessarily bite Julia the first night but that it would bite her sooner or later. He had bolted the bed to the floor so that it could not be moved away from the ventilator and the bell-rope. He had trained the snake to come back up the bell-rope and back through the ventilator when he blew on a whistle. Evidently he always blew the whistle at three o'clock in the morning to make sure the girl didn't wake up and find the snake in bed with her. Dr. Watson does not say as much, out of Victorian delicacy, but it would appear that the snake, a native of a tropical country, must have crawled right under the covers and curled up beside the warm body of the sleeping girl. Julia told her sister than she had heard the whistle on three consecutive nights. It was on the fourth night that she was bitten and died in her sister Helen's arms in the corridor mumbling incoherently about a "speckled band" and pointing at her stepfather's room. Julia had a charred match in one hand and a matchbox in the other. Evidently she heard the whistle, turned over in bed to reach for the matches, and rolled right on top of the deadly snake sleeping peacefully beside her.
Julia was engaged to be married. Dr. Roylott's motive for killing her was to avoid having to pay her a large annual sum of money in accordance with the provisions of her dead mother's will. Before going down to Stoke Moran, Holmes goes to Doctors' Commons to look up Helen's mother's will. When he returns to Baker Street he tells Watson:
"The total income, which at the time of the wife's death was little short of £1100, is now, through the fall in agricultural prices, not more than £750. Each daughter can claim an income of £250, in case of marriage. It is evident, therefore, that if both girls had married, this beauty would have had a mere pittance, while even one of them would cripple him to a very serious extent."
Now Helen is engaged to be married and Roylott wants to kill her for the same reason. Helen wakes up when she hears the same low whistle her sister described to her two years ago. Helen waits until daylight and comes to London immediately to ask Sherlock Holmes for his help and advice. The reason Dr. Roylott gives for moving Helen into her deceased sister's bedroom is that he wants to have some repair work done to Helen's own bedroom, which is on the other side of the bedroom she is temporarily occupying and not accessible to the snake. In the course of her long back-story, Helen tells her reason for sleeping in Julia's former bedroom as well as her reason for coming to consult Sherlock Holmes at this early hour:
"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. I sprang up and lit the lamp, but nothing was to be seen in the room. I was too shaken to go to bed again, however, so I dressed, and as soon as it was daylight I slipped down, got a dog-cart at the Crown Inn, which is opposite, and drove to Leatherhead, from whence I have come on this morning with the one object of seeing you and asking your advice.”