In the Adventure of the Speckled Band, why does Helen move into Julia's room? What frightens her when she does that?

Before Julia died she had told her sister Helen about
hearing a strange whistling but could not tell exactly where it was coming from. Helen
explains to Holmes and Watson why she is presently occupying the room where Helen died
and sleeping in Helen's bed.

readability="16">

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.

When Holmes inspects
the three adjoining bedrooms at Stoke Moran, he remarks that the extensive repairs to
Helen's bedroom seem totally unnecessary, which suggests that Dr. Roylott was only
having them done in order to force Helen to move into the only other available bedroom,
which is Helen's former bedroom and is situated right next to Roylott's. Helen is
already understandably frightened at having to sleep in the room where Julia died and to
sleep in her sister's bed. But then when she hears the "low whistle" Julia told her
about she becomes terrified. This whistling is what prompts her to travel to London
early the next morning and consult Sherlock
Holmes.

Evidently what Dr. Roylott is doing is sending the
snake through the ventilator late at night when he can be sure Helen is asleep. But he
has to call the snake back through the ventilator before daylight. Otherwise Helen might
wake up early and be able to see the snake. So Roylott has trained the snake to return
when it hears the furtive whistle. It is given a saucer of milk as an inducement to
return. (This sounds very much like Pavlovian
conditioning.) 

Doyle does not say much about what the
snake does after it slips through the ventilator and slides down the dummy bell-rope
onto the bed beside the sleeping girl. The snake would not be motivated to bite the girl
without provocation. There is a strong implication that this swamp adder, coming as it
does from a hot climate, would be seeking warmth and might actually crawl under the
covers and curl up beside the girl's warm body. It is mentioned many times that the
weather is very cold. Helen mentions that it was also a cold night when her sister
died.

It was a
wild night. The wind was howling outside, and the rain was beating and splashing against
the windows.

Therefore the
snake would probably not try to escape from captivity, although it might at least be
possible for it to crawl under the bedroom door. The most likely way in which the girl
would be bitten would be if she turned over in her sleep and ended up lying right on top
of the loathsome speckled reptile. Then it would surely bite her through her nightgown.
That was probably what happened to Julia two years earlier. Julia may have been sleeping
with a poisonous snake beside her for a number of nights before the snake finally
struck.