The teenager jailed for sexually abusing siblings put in his parents' care by Child Youth and Family was also accused of raping his 9-year-old cousin.

And the Herald can now report on her case, including the heartbreaking letter she wrote her mother to disclose the alleged abuse.

The Herald revealed last week that a 17-year-old boy had been jailed for more than four years for repeatedly sexually abusing two children who had been put in his parents' care by CYF, now known as the Ministry for Children, Oranga Tamariki.

The victims were a 7-year-old girl and her 8-year-old brother.

But her father - the teenage offender's uncle - has spoken out today about the case. 

The judge who presided over the District Court sentencing - moved from the Youth Court due to the seriousness of the charges - has permitted publication of the facts.

But strict suppression orders are in place around the offender to prevent publication of his name - which could identify his victims and alleged victims.

Police alleged that the boy raped, sexually violated and indecently assaulted his cousin some time between February 2013 and February 2014.

They told the Youth Court that in mid-2016 the girl, then aged 12, left a note for her mother on her handbag disclosing the alleged abuse.

The note read:

To mum,
I am writing to tell you about what happened sometime three years ago at nana's house.
(The youth) told me about sex and raped me. I did not know what was happening at the time but I knew it was wrong.
I have been too chicken to speak up about it until recently. He still asks to see my boobs repeatedly.
I just thought I should tell you about this.

Inside the fort, he allegedly asked her if she knew "about the birds and the bees" and if she "wanted to do it".

He then allegedly took her clothes off, undressed himself and sexually assaulted her.

The Herald has chosen not to detail the specifics of the alleged abuse to protect the victim. 

She said part of the first alleged assault happened in a wardrobe in the bedroom.

After the alleged assault the girl claimed the youth repeatedly walked in on her as she was getting dressed after showering at their grandparents' home.

On one occasion she said he asked her "do you remember what we did in the closet?".

When spoken to the police the teenager denied assaulting his cousin, and later pleaded not guilty to three charges in the Youth Court. 

After hearing all the facts of the case, including evidence from the girl, the judge dismissed the charges. 

The girl's father said that was "devastating".

It was even more painful for him and his family when he read the details of the other offending in the Herald.

They had not known about the CYF children until details were published. 

"We knew there were other cases, but we hadn't been told the details," he said.

"This has been an unbelievably difficult situation."

The man said there was no question in his mind that his daughter had been abused and he was disappointed with the outcome of the court case. 

"This has been absolutely horrendous, but we are so proud of her," he said.

"We are proud of the fact that she stood up, spoke up in the first place.

"She gave evidence … it was devastating, absolutely devastating to hear a judge say he could not find him guilty."

He said the case was "incredibly harrowing" and stressful for his daughter, and the rest of the family.

"It's been extremely upsetting for my daughter, as you can imagine," he said. 

"She has had counselling, but she said she's not going back, that she's finished - she just wants to move on."

The man was pleased the teen had been jailed for his offending against the CYF children.

But he was concerned about him offending in future. 

"I believe there is absolutely no chance of rehabilitation, whatsoever," he said. 

"That's why we decided to go through with the court process instead of handling this in-house like some of the family wanted.

"We wanted to stop him from being able to do this again to someone else.

"I just don't ever want him to ever be in a situation where he can ever do this again."