Illawarra siblings repeatedly raped and molested by their stepfather for more than a decade have spoken of the deep mental scars they still carry from the abuse.

The brother and sister, united in their anger and grief, on Friday stood just metres away from their attacker in Wollongong District Court and tearfully described the terrible toll his actions had taken on them.

"He was meant to be my new father when my parents split but instead he made me wish I was dead," the boy, now 18, told the court. "He made me his prisoner."

The stepfather, who cannot be named for legal reasons, began abusing his stepson in 2001 when the boy was just six years old, forcing him to engage in sexual acts "about once a month" over the space of 10 years.

He also molested his stepdaughter and natural daughter, coming to treat the former like a girlfriend when she was aged 16.

The ongoing abuse finally came to light in 2011, when both teens independently told family members of their stepfather’s actions.

The man’s natural daughter, aged 11 at the time, disclosed her abuse to police soon after.

Her father was arrested in October and charged with a string of child sex offences. He pleaded guilty to the matters in September. 

In court on Friday, the boy said his stepfather had sexually abused him "whenever he had the chance", but admitted the physical pain was "nothing compared to what comes at night".

"The mental scars still remain ... I’ve lost count of how many times I almost killed myself," he said.

The boy’s sister told the court her stepfather had controlled her "like a robot, doing all the dirty work for him".

"As long as he was pleasured [that] was all that mattered to him," she said. "I was treated and abused like a sex toy, it seemed like a game to him."

The girl, now aged 21, said the man’s actions ruined her relationship with her mother at the time.

"He divided our [family] so his dirty little secret wouldn’t come out," she said, but noted her relationship with her mother now was stronger than ever.

The man, who the court heard had himself suffered abuse at the hands of his uncle when he was a young child, will be sentenced later this month.