In "The Adventure of the Speckled Band," why had Helen Stoner only heard the whistle the night she was in her sister's bed and not sooner?

I think you may have slightly confused what happens in
this story. Helen Stoner actually hears the strange whistle at night twice before she
comes to Sherlock Holmes. The first time she heard it, as she narrates to Sherlock
Holmes and Dr. Watson, was the night when her sister died. After hearing her sister's
cry, she rushes out to go to her, and note what she says
happened:

I
sprang from my bed, wrapped a shawl around me, and rushed into the corridor. As I opened
my door I seemed to hear a low whistle, such as my sister described, and a few moments
later a clanging sound, as if a mass of metal had
fallen.

Previously, Helen's
sister had asked Helen if she had heard a whistle during the night. Thus it is that
Helen associates this whistle with her sister's death and the terror that she evinced
before her final breath. Of course, now that Helen herself has heard the whistle at
night, she shares that same terror of the mysterious fate that befell her sister, and
has come to Sherlock Holmes to ask him to investigate.