Summary :
It's the story of a young woman (Helen) who comes to ask Sherlock Holmes for assitance, after the death of her twin sister Julia, who died mysteriously two years ago before her marriage.
At the time, they both lived with their father-in-law in his manor. Just before her death, Julia told Helen that she had heard strange noises in her room. These same noises that Helen hears today while she is preparing her marriage. She also informs Sherlock Holmes that her father-in-law asked her to go to the Julia's room, pretending that he had to work in her room. Sherlock and Watson decide to spend the night in the room where Helen is supposed to sleep while she spends the night in an inn.
During the night, they hear this famous noise which proves to be the hiss of a snake that the faher-in-law let in throug an air-vent communicating with his room to kill Helen. Sherlock succeeds in getting rid of the snake flee but it bites the father-in-law who eventually dies.
To conclude, if the two sisters had got married, they could have claimed their share of money that their mother had bekueathed to them in her will and therefore the father-in-law would be ruined and that's why he wanted to kill them.