Julia was engaged to be married at the time of her death, and now Helen has become engaged.
"Julia went there at Christmas two years ago, and met there a half-pay major of marines, to whom she became engaged."
"A month ago, however, a dear friend, whom I have known for many years, has done me the honour to ask my hand in marriage."
Before her death, Helen spoke to her sister about hearing a strange whistling sound in the early morning, and now Helen has heard the same sound. Helen believes the sound was somehow connected with Julia's death and that it may represent a threat to her own life. This is why she is so terrified and why she has come to see Sherlock Holmes.
“‘Because during the last few nights I [Julia] have always, about three in the morning, heard a low, clear whistle. I am a light sleeper, and it has awakened me. I cannot tell where it came from—perhaps from the next room, perhaps from the lawn. I thought that I would just ask you whether you had heard it.’"
"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."
Helen is now occupying the room where Julia died two years ago. Her stepfather Dr. Roylott has ordered some repairs done on Helen's room, forcing her to move into the bedroom right next to his. There is a ventilator between these two rooms, and the bed Helen sleeps in is bolted to the floor to prevent it from being moved.
"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."
Dr. Roylott, of course, wants to kill Helen in the same way he killed Julia two years earlier, and he intends to do it in the same manner, by sending a poisonous snake through the ventilator and down the dummy bell-rope onto Helen's bed. He killed Julia to avoid having to give her one-third of her deceased mother's money, as he would have been legally obligated to do if she got married, and he is trying to kill Helen by the same means for the same motive.