In The Adventure of the Speckled Band why did Helen Stoner hear whistle when she did?

In "The Adventure of the Speckled Band," Helen Stoner hears the whistle because it was a signal used by her step-father, Dr. Roylott, to alert his Indian snake.

Here's the breakdown:

* Dr. Roylott's Motive: He wanted to inherit Helen's money and saw her as an obstacle. He planned to kill her with a poisonous Indian snake.

* The Whistle: The whistle was a method Dr. Roylott used to signal his snake. He trained the snake to respond to the sound.

* The Timing: Helen heard the whistle just before her sister's death, suggesting it was the signal to release the snake. This aligns with the snake entering the room through the ventilation shaft.

The whistle, therefore, is a crucial element of Dr. Roylott's plan and a key piece of evidence that helps Sherlock Holmes solve the mystery.