The man, who was under 18 the first time he sexually assaulted her and who cannot legally be named, fronted the ACT Supreme Court on Friday where he was sentenced for his crimes.

He had earlier pleaded guilty to three counts of raping a child under 16 and one count of an act of indecency on a child under 10.

Justice Chrissa Loukas-Karlsson set a non-parole term of 18 months and he will be eligible for release in December 2022.

"The court recognises that you are a different person today than the person who committed the offences and you have rehabilitated yourself," she told the now 31-year-old.

"Your rehabilitation does not erase the past but what it does is impact is on the non-parole period."

The court's public gallery was full of the man's and the victim's supporters.

The court heard how the now engaged father started sexualising the girl in 2002.

In 2004, when the victim was 9 and the man 15, he had her lie down on the couch and pulled down his pants and rubbed himself on her while his infant niece was nearby.

She didn't tell anyone; by this time the sexualised behaviour had become normalised, the court heard.

Years later, when he was 20 and she was 14, he apologised to her and said he regretted what he did but could not explain it.

He raped her again after that, going into the bedroom where she was staying and holding her arms down.

Another time they were again at their grandparents house, he took his cousin on a drive and parked near a mountain in Woden. He pushed her up against a rock and raped her.

It was during this assault that the woman, then 15, became pregnant.

When she found out she was pregnant, she came up with a story that she had been raped by a stranger but she was scared and told police she didn't want an investigation.

She had an abortion, and testing later proved it was her cousin involved.

He raped her once more after the time in Woden, the court heard.

She confided only in her little brother, who later died by suicide, who had raped her.

It was only later that he told their mother who had been abusing his sister, and she went to her daughter and they agreed to again speak to police.

In a victim impact statement the woman described feeling "powerless".

"You took advantage of me in an indescribable, disgusting way," she said.

She said her cousin knew what he did was wrong but sexually abused her at every opportunity.

"I'll never forget what you did and I'll never, ever forgive you."

References from the man's family said he was an honest and hardworking father who had overcome a traumatic childhood full of abuse and violence.

They said he shouldn't go to jail because he had already rehabilitated himself.

"The man I know isn't that young, damaged boy anymore," his mother-in-law said.

He wrote a letter of apology, accepting responsibility for his crimes and acknowledging the hurt he had caused his cousin and his family.

He said he was full of shame.

"I assure you I will never reoffend or be before the courts again."

Our journalists work hard to provide local, up-to-date news to the community. This is how you can continue to access our trusted content: