A married couple allegedly plied their own children with drugs and subjected them to years of systematic incestuous sexual abuse.

Jurors at Truro Crown Court yesterday heard a summary of witness evidence including allegations that a father sexually assaulted his own son.

The defendant's husband has already admitted several of the charges.

Evidence from the boy and his stepsister was read out in the trial of the female defendant, who has denied 13 charges, four of which related to supply of drugs.

In order to protect the identities of the alleged victims, none of the family members will be named in this report.

Prosecuting, Heather Hope said that the male victim claimed that his father and stepmother fostered an environment in which taking illegal drugs, watching pornography and touching each other's naked bodies was completely normal.

He said that as an adult he now looks back at his childhood and realises he was being groomed for sexual activity.

The boy's stepmother allegedly began having sex with him when he was just 11, and stopped when he was around the age of 14.

Miss Hope said that the first time it happened, the woman showed the boy her genitals before having sex with him while his father was in the same bed.

In another alleged incident, the boy had sex with his stepmother while his father encouraged him to pretend it was his own biological sister.

The boy's stepsister also alleges that she was abused by the pair from the age of seven.

She said that she woke up to find her stepfather sexually assaulting her, which she says he continued to do for years.

On another occasion, she alleged that she was having trouble sleeping and was told by her mother to masturbate to help get to sleep.

Miss Hope added that the girl was also shown how to use a vibrator by her mother, who used it herself before giving it to her daughter to use.

Another charge details how the woman and her husband had sex in front of the girl for the purpose of obtaining sexual gratification.

She says it was not until the age of around 12 that she became aware of what is sexually acceptable in families, at which point the abuse stopped.

The trial continues today.