A NORTH-WEST Tasmanian man who started raping his four-year-old sister when he was 12 has been given a suspended jail sentence for his sexual crimes.

The man, who is now 22 and cannot be named, pleaded guilty in the Supreme Court in Burnie to a charge of maintaining a sexual relationship with a young person, The Mercury said.

He was sentenced to two years in prison but the term was suspended on condition he commit no offences for three years.

Justice Alan Blow also ordered that the man's name be placed on the community protection register for seven years.

The court also heard the man had pleaded guilty to sexually assaulting his stepmother in 2006 and was given a suspended sentence under the Youth Justice Act.

Justice Blow said the man had started a sexual relationship with his little sister when she was four or five and he was 12.

The assaults stopped when the girl was seven.

The court heard the girl was raped at least once a week over that time.

"She was too young to understand what was happening to her," Justice Blow told the man during sentencing.

The girl is now 14 and Justice Blow said the crimes had had a devastating impact on her life.

"The rapes were painful and she has nightmares and is still receiving counselling," he said.

Justice Blow said the man had endured a traumatic childhood and adolescence and was repeatedly physically abused.

His mother died of cancer when he was a child and he was exposed to sexual material at a young age but little parental control.

Justice Blow said the man has undertaken a treatment program for sexual offenders and had made progress but there was still a moderate to low risk he would reoffend.

"Prison, while well deserved according to one point of view, could be counter-productive," Justice Blow said.

"He has a good job, no alcohol or drug problems and has been living in a stable environment with relatives since 2007.

"He is extremely remorseful and realises he has harmed his sister.

"He is not the same person as the troubled individual who abused his sister and has not offended since 2006."

Justice Blow said a suspended sentence was not the ideal punishment.

"I would have liked to impose a community service order but that office was unable to make arrangements," he said.

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