The 29-year-old is the son of a husband and wife whose abuse of their two daughters landed them in jail, where they are serving lengthy sentences.

Now the ACT Supreme Court's acting Justice John Nield has sentenced their son to 20 years, with a non-parole period of 15 years - the same length as his father.

The man broke into a former partner's home, while her children were asleep, and subjected her to an ordeal the judge yesterday described as ''appalling in the extreme''.

It was a crime mirroring another which led him to be jailed in NSW years earlier.

In 2004 he unlawfully confined a former partner and raped her, earning him two years and nine months behind bars before his release on parole in 2008.

The Canberra Times has chosen not to name the offender, in order not to make the connection to his parents.

Their names were suppressed in previous court proceedings in order to protect the anonymity of their daughters.

But it can be revealed that both parents are serving long sentences for the prolonged sexual abuse of their daughters in the 1980s and 1990s.

Chief Justice Terence Higgins, sentencing the pair in September 2010, said the crimes ''strike at the very foundation of civilised society''.

The sustained molestation only ended when the girls were old enough to escape from what the judge called an ''evil situation''.

''Those acts have seriously damaged and scarred these two young women, who were his daughters,'' he said at the time. ''They are and were innocent victims.''

Their father, ''the instigator of profoundly evil behaviour'' pleaded guilty to a string of offences including incest and was jailed. His wife was jailed for 10 years, with a non-parole period of five years.

The dark legacy of the couple was detailed in a pre-sentence report tendered in their son's sentencing proceedings.

''[The offender] was raised in a socially dysfunctional environment, dominated by his father's volatile and often alcohol-fuelled outbursts, undermined by his mother's inability to extricate herself or her children from the tyranny of the systemic psychological abuse of them all, and ultimately the protracted sexual abuse of his two sisters,'' the report states.

The court heard how he grew up to be a highly intelligent man, but with a history of alcohol and cannabis use.

But he was also described as a ''gross and ever present risk upon any release to the community'' without further treatment.

In July last year, the son broke into his victim's home.

She saw him standing over her with a knife which he held to her throat, warning her not to scream or struggle. He was subsequently charged with seven counts of having sexual intercourse without consent and a charge of sexual assault in the third degree, threatening harm in order to have sex with a person.

The defendant was also charged with unlawful confinement, assault, aggravated burglary and committing an act of indecency without consent.

He pleaded guilty to the offences at an early state in the Magistrates Court, and the judge took that into account on sentence.

But Justice Nield said he didn't believe the defendant was genuinely remorseful for his crimes. He said the offender did what he did to exercise control over the victim, to teach her a lesson for ending their relationship and to obtain perverse and sadistic sexual gratification without regard to her feelings or wellbeing.

The man will be eligible for parole in late 2026.