Dr. Roylott's first murder was of Helen Stoner's twin sister Julia two years before Helen comes to consult Sherlock Holmes. When Helen tells her long back story to Holmes and Watson she does not suspect her stepfather of being responsible for her sister's death. The factors that allowed Dr. Roylott's murder of Julia to go undetected are all contained in a single paragraph in Helen's story, although she does not understand their significance as she tells it. The county coroner held a thorough investigation of Julia's death.
“He investigated the case with great care, for Dr. Roylott's conduct had long been notorious in the county, but he was unable to find any satisfactory cause of death. My evidence showed that the door had been fastened upon the inner side, and the windows were blocked by old-fashioned shutters with broad iron bars, which were secured every night. The walls were carefully sounded, and were shown to be quite solid all round, and the flooring was also thoroughly examined, with the same result. The chimney is wide, but is barred up by four large staples. It is certain, therefore, that my sister was quite alone when she met her end. Besides, there were no marks of any violence upon her.”
The factors that allowed Dr. Roylott's murder of Julia to go undetected are mainly (1) that Julia was undoubtedly all alone in a locked room when she was stricken, and (2) that there were "no marks of any violence upon her." The venom of the poisonous snake that Dr. Roylott used for the murder is undetectable, and apparently the marks of its fangs were too small to be noticed.
Evidently this "speckled band," or Indian swamp adder, crawled through the ventilator and down the dummy bell-rope onto the bed right beside the head of the sleeping girl. The author, Arthur Conan Doyle, indicates many times that the weather is very cold. The snake would not try to escape from its captivity because it comes from a warm, humid climate and would not attempt to venture out into the cold night. Instead, it would be more likely to crawl under the bed-covers and curl up right beside the sleeping girl's warm body. So Julia had been sleeping with a snake for at least several nights before she was finally bitten. Dr. Roylott knew that it was just a matter of time before Julia would make some movement that would cause the snake to bite. When that happened the snake would bite her body through her nightgown. The examiners might never even have removed her nightgown, or if they did so they overlooked the small bite marks. Helen tells Holmes:
"In her right hand was found the charred stump of a match, and in her left a match-box.”
It would seem that Helen heard the low whistle again on the fourth night. She rolled over in her narrow bed to reach for the match-box, and in doing so she rolled right on top of the snake curled up beside her.
The same thing would have happened to Helen if Julia had not told her about hearing a strange low whistle for several nights in a row. When Helen hears that whistle on her first night in the Julia's former bed, which is bolted to the floor, she goes to see Sherlock Holmes the very next morning in a panic.
This story is a so-called "locked-room murder mystery." The prototype may have been Edgar Allan Poe's "The Murders in the Rue Morgue." The main reason Dr. Roylott could not be implicated in Julia Stoner's death was that she was undoubtedly all alone in her room with her door locked and the shutters tightly bolted.