But a judge said the girl, then 12, had been a willing participant in their encounters after she developed a crush on her half-sibling when they met for the first time in 2013.

The man pleaded guilty to charges of incest, using a carriage service to groom a child and using a carriage service to transmit indecent material.

Justice Hilary Penfold handed him a three-year good behaviour bond in the ACT Supreme Court on Thursday.

The offences took place in 2013 when the man, 20, met the girl for the first time when he came from interstate to stay with their shared father.

The man molested the girl one night when they shared a bed at their father's Canberra home.

He later sent her text messages of a sexual nature saying he could teach her about "dirty stuff", and the pair had phone sex three times between July and August.

The court heard the victim had developed a crush on her half-brother after he came to live with their father and had largely been an "enthusiastic" participant in the encounters.

The victim wrote about the offences in her diary and later told a friend, whose mother referred the matter to protective services.

The court heard the man had been sexually assaulted as a child and his father and step-father had been violent towards him.

He had been bullied and had trouble maintaining friendships.

The man later developed an addiction to online gaming, which often saw him play for days at a time without a break, and he considered his fellow gamers to be his most significant relationships outside his family.

Ms Penfold said the man lacked the maturity level expected in someone his age, but had expressed remorse for his actions and acknowledged during counselling the impact they could have on the girl's future relationships.

Victim impact statements said his half-sister had suffered depression and anxiety after the incidents.

In handing down her sentence, Justice Penfold took into account the girl's willingness to participate in the acts because she had a crush on the man, but said he had a responsibility to resist.

Justice Penfold said the man knew what he was doing was wrong and had taken advantage of the girl because he was lonely, depressed and lacked self-confidence.

She said his age, immaturity and social awkwardness would make him "extremely vulnerable" in a prison setting.

Justice Penfold sentenced the man to three-and-a-half years in jail, to be fully suspended upon entering into a good-behaviour order.

She also ordered he complete 280 hours of community service and to take part in the territory's adult sex offender program if he was suitable.