Why would Dr.Roylott be opposed to his stepdaughters marrying?
After talking with Helen Stoner for a long time in the early morning, and after the tumultuous intrusion of her stepfather Dr. Roylott, Holmes goes to Doctors' Commons to do some research. When he returns he tells Dr. Watson:
“I have seen the will of the deceased wife,” said he. “To determine its exact meaning I have been obliged to work out the present prices of the investments with which it is concerned. The total income, which at the time of the wife's death was little short of £1100, is now, through the fall in agricultural prices, not more than £750. Each daughter can claim an income of £250, in case of marriage. It is evident, therefore, that if both girls had married, this beauty would have had a mere pittance, while even one of them would cripple him to a very serious extent. My morning's work has not been wasted, since it has proved that he has the very strongest motives for standing in the way of anything of the sort."
The big house at Stoke Moran is ancient and dilapidated. It is also heavily mortgaged. Under the terms of his deceased wife's will, Roylott would have been forced to pay Helen's sister Julia one-third of the annual income of the estate every year, although he could control the capital himself. That would have been approximately £250 a year. Rather than pay out that amount to Julia, who was soon to be married, Roylott had killed her with his poisonous Indian swamp adder, the "speckled band" Julia spoke of to her sister as she lay dying in her arms. Now Helen has become engaged and Dr. Roylott has arranged some unnecessary repairs to her bedroom so that she has been forced to move into the room formerly occupied by her sister.
Shortly before Julia died, she asked her sister if she had ever heard a strange low whistle in the middle of the night.
... during the last few nights I have always, about three in the morning, heard a low, clear whistle. I am a light sleeper, and it has awakened me.
Evidently Julia heard the whistle for three nights in a row and died on the fourth night. Helen had been sleeping in Julia's room for two nights. On the second night:
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.
As soon as it is daylight, Helen comes to London by train and tells Holmes and Watson her long back-story. She arrives at 221B Baker Street very early on a cold morning. By late that night Holmes has solved the mystery and Dr. Roylott is dead, bitten by his own snake. Holmes not only saves Helen from being murdered in her bed, but he solves the two-year-old "locked-room murder mystery" of Julia's death.
When Holmes inspects the room now occupied by Helen, he notes the dummy bell-pull, the ventilator between that room and Dr. Roylott's adjacent room, and the fact that the bed is fastened to the floor so that it cannot be moved. As Holmes later tells Watson when they are discussing the case:
'The idea of a snake instantly occurred to me..."