The woman, who cannot be named, read out a victim impact statement in the Launceston Supreme Court, detailing the assault to which her now 52-year-old brother has pleaded guilty.

The two counts of indecent assault date back to the 1980s, when the pair - raised in a deeply religious household - were living in Launceston with their family.

The complainant was 12 at the time, while the accused, who also cannot be named, was about 18 and studying at the University of Tasmania's Newnham campus.

One night, when their parents weren't home, the complainant said her brother led her to the attic.

She told police in 2016 that the pair were naked in the attic and she performed oral sex on her brother.

The accused also laid on top of the complainant and masturbated.

The complainant said she never wanted to be in that situation and felt "disgusted".

"Our family looked like any other family - behind closed doors, it was a different story," she told the court.

"I grew up never knowing what innocence was ... never knowing what real love was."

The complainant kept the abuse to herself for many years, until she told her husband and reconciled with her brother with the help of a Christian counsellor.

But she said she eventually came to the realisation that "if the perpetrator was to understand the true nature of his actions, he had to face the consequences".

And so in 2016, she reported the assault to police.

"There are many who blame me [in the church]," she said. "My mum has shed many tears - but her tears are for [my brother].

"I dared to speak the truth, yet she and others don't believe me.

"It's exactly what I feared would happen."

The accused, who represented himself, told the court he had been married for 29 years and had five children.

He said he was responsible for the care of his 73-year-old mother.

"I will never be able to undo what I have done," he said.

"I accept full responsibility for my actions [and] I've never tried to defend myself.

"I've always told the truth to the best of my knowledge."

The accused said he felt "sincere remorse".

He was bailed until sentencing on August 21 in the Hobart Supreme Court.