I actually don't think that Helen Stoner was aware that she was in grave danger when she went to visit Holmes. She went simply due to the low whistle which she heard in the middle of the night she had to spend in her sister's room.
It is Holmes who suspects that there is a danger to her life and makes an appointment to visit her soon so that he may investigate the cause behind the whistle she heard. Holmes makes a plan to discover the reality that night and gives strict instructions to Ms. Stoner. She is to return to her old bedroom while Holmes and Watson enter the room in which she heard the whistle and wait.
Eventually, the whistle sounds and a snake enters the room through an air ventilator from the father's room next door with the use of a rope that had been placed on top of the bed. Holmes hits the snake which in fear returns into the ventilator and ends up biting the father. The snake is revealed to be the most poisonous snake from India.
Although Helen Stoner came with the feeling that there was something strange taking place in her home, it was really Holmes who understood the seriousness of the matter and the risk to her life thanks to his keen sense of reasoning and intuition.
Helen Stoner seeks Holmes' help because she fears for her life as she is convinced that she will die just like how her sister died. Helen and her sister Julia live with their stepfather Dr.Roylott. Two years ago Julia got engaged and just a fortnight before her wedding she died a terrible death under mysterious circumstances screaming, 'Oh, my God! Helen! It was the band! The speckled band!' She had informed Helen earlier that she often heard a 'low clear whistle' at three in the morning.
Now, Helen is in a similar situation. She has been engaged to one Percy Armitage and is to be married in the coming Spring season. But two days ago some repairs were started in her room and she has been compelled to stay in the very same room in which her sister died. Last night when she was sleeping on the same bed in which her sister met her tragic end,to her horror, she heard the same low whistle which signalled her sister's tragic death. So, in sheer fright she has taken the first train to London and rushed to meet Sherlock Holmes to seek his advice as to how to save her life from impending death.