The man, who is not named for legal reasons, is also accused of committing similar acts on his niece, who has kept a handwritten diary since the 1980s about the alleged offending.

The man appeared in the ACT Supreme Court for trial on Monday when he pleaded not guilty to two counts of committing an indecent act on a child.

The court heard the alleged offending against his sister was in 1971-75 and and his niece in 1979-83 at his Canberra premises during the complainants' school holidays.

The prosecution case in relation to the first complainant is that the accused touched her vagina and genital area a number of times during her stays.

The case also includes tendency evidence involving uncharged acts by the accused - that he allegedly touched the complainant in three incidents, including a family gathering, while they were in NSW before the charge period that is the subject of the trial.

Played to the court was her evidence-in-chief interview with police in July 2020.

She said she remembered her brother coming into her bedroom "in the middle of the night" when he allegedly "put his hands into my underpants ... and just touch me".

"This went on most times when I visited ... and I did visit a number of times," she said.

The complainant alleges her brother "was always too familiar and too sexually inappropriate with me", including possibly up until she was 16.

She said she once told him to "stop touching me, you have to leave me alone".

She said she did not show any signs the alleged course of conduct "affected me in any way" for many years and she repressed a lot of her anger.

"I was adopted ... I suppose for many years, I thought the reason why it had happened to me was because I wasn't the real deal," she said.

"This is what I deserved, so to speak."

In her police interview, she also spoke about the uncharged acts in NSW, including that after he allegedly touched and kissed her, he said words to the effect of "now it's your turn to kiss me".

"I don't know whether I did," she said.

The complainant, however, said she gets "a bit confused sometimes between when certain things happen" and that her brother was "very loving towards me".

"I thought the world of him," she said.

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In his opening statement, Chief Crown prosecutor Anthony Williamson SC alleged "the only available inference is that this was done for the accused's sexual gratification".

Mr Williamson said the first complainant's husband will also give evidence, including about a phone conversation with the accused, who said: "I don't recall doing anything, but if I did, I'm sorry."

The prosecutor said the case in relation to the second complainant was "strikingly similar" as she also stayed at the accused's premises during her school holidays.

The court heard evidence would include her handwritten diary, which she has kept since the 80s.

One entry reads: "I think it is because of something that happened to me when I was only little. I can't say what it is just in case someone reads it. It has been bottled up inside me for so long and I just feel like yelling it out to the world, but I know I can't. Anyway, I've said too much already."

Mr Williamson said that complainant and her close friend in the 80s wrote song lyrics that included "touching me" and "don't touch me there".

He said the close friend then questioned her about them, saying words to the effect of "what's that crap all about"?

"At that point, the complainant burst into tears and told her friend [about the allegations]," Mr Williamson said.

He said a doctor would also testify about the delay in reporting the allegations, saying the court should expect evidence about it being more common for alleged victims of child sexual abuse to delay making complaints.

Defence lawyer Travis Jackson chose not to make an opening statement. The judge alone trial is expected to take until Friday.

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