Speaking quietly and often crying, she described how on one occasion she had been strapped to a frame and raped by Mr West and how later she was assaulted by the couple after a friendly night out.
She was told they were helping her future sex life, that she was lucky to have such caring parents and that this type of behaviour went on in all families.
Mrs West, 41, denies murdering 10 girls and young women whose remains were found at 25 Cromwell Street, Gloucester, and at the couple's previous home in the city. Mr West, who was charged with 12 murders, was found dead in his prison cell on New Year's Day.
Mrs Davis, 31, told the court that she was the daughter of Mr West and his first wife, Rena, but she was brought up by Rosemary West.
She said that when she was about eight her father and stepmother took her down into the cellar at 25 Cromwell Street where Mrs West undressed her.
Mrs Davis said: "I was crying, just asking what is going on? What is happening? And I was told I should be very grateful and I was very lucky that I had such caring parents and they were going to help me make sure that when I got married I would be able to satisfy my husband. I was struggling . . . so much that I had my hands bound and I was gagged.
"I remember the excruciating pain. I just wanted the hurt to go away. I also felt I should not be so ungrateful because they were doing this to help me."
Brian Leveson QC, for the prosecution, asked what Mrs West's reaction had been. She replied: "She was laughing and smirking and joining in and just saying to me it was for my own good and to stop being silly."
Mrs Davis said Mrs West scratched her chest and warned her not to tell anybody what had happened. She told the jury of eight men and four women that later her father brought home a U-shaped metal frame he had made. In the cellar she said she was stripped and strapped to the frame and when she started screaming she was gagged. Her father had sex with her and Mrs West then sexually abused her.
She said that her stepmother was present on a number of occasions when Mr West had sex with her.
When she was 13, Mrs Davis said she went out to a pub with her stepmother and they had a sociable evening. Mrs West made her drink barley wine and she was drunk when they left. Mrs Davis said: "Rosemary said we would be walking home. I started walking. All of a sudden my dad drove up and I was pushed in to the back of the van.
"My father came into the back. He was hitting me and Rosemary was being sarcastic and calling me names and kept hitting me and my father had sexual intercourse with me. She was just laughing. I didn't understand what was happening."
Mrs Davis said that when one of her teachers noticed that she was bruised on one occasion "a lady from the welfare" came round to 25 Cromwell Street. After she left Mrs West gave her "the biggest hiding" of her life.
She said that Charmaine West, Mr West's first wife's daughter by another man, had antagonised Rosemary West and had refused to cry when she was beaten as she regarded this as a sign of weakness.
Mrs Davis added that when Mrs West told her that Charmaine had gone to live with her mother in Scotland Mr West was in prison at the time. The prosecution alleges that a child's remains found at the West's previous home at 25 Midland Road, Gloucester, are those of Charmaine.
She also said that Mr West had taunted Mrs West about the fact that Shirley Robinson, a lodger at number 25, was expecting his baby. The remains of Shirley and an unborn child were found at the house.
Mrs Davis continued: "My stepmother was very upset and it was understandable, my father was humiliating her. After that it became very tense in the household. There were a lot of rows between my stepmother and my father.
"I came home from school one day very close to when Shirley's baby was due and I was told that Shirley had gone to Germany." She said that she never saw Shirley again.
Asked about the relationship between her father and stepmother, Mrs Davis said: "I believe they always told each other what they were doing. They had total trust."
Today the jury will visit 25 Cromwell Street.Reuse content