Rappel, who is serving a 32 year jail sentence for killing his former partner Tara Costigan with an axe, had first pleaded not guilty to the bashing before switching his plea mid-trial.

The court on Monday heard the two brothers had been bashed in their cells at the Alexander Maconochie Centre on August 7 last year.

The younger brother was admitted to hospital with 36 individual injuries, including a collapsed lung and fractured ribs and facial bones.

The elder was also taken to hospital so a tear injury to his ear could be stitched up.

At first, Rappel and four other men went to an ACT Supreme Court trial over the bashing.

All five accused - Rappel, 43, Alex Bourne, 30, Daniel Grech, 28, Jeremy Leigh Jackson Reid, 26, and Isileli Paea Taholo, 31 – pleaded not guilty to two counts each of intentionally inflicting grievous bodily harm.

But on Tuesday, mid-trial, Rappel switched his plea to guilty and took responsiblity for the bashing. Grech, who was captured on camera landing a punch, also pleaded guilty to a common assault.

The Crown declined to proceed with the charges against Mr Reid, Mr Bourne and Mr Taholo.

Justice Michael Elkaim said in relation to both men that while they may be in prison, the intent of authorities is to deprive the victims of their liberty and not that they be susceptible to attack.

"I do however note that in the case of [one victim], the evidence he gave during the trial would suggest that his own approach to the situation would have concentrated on fight rather than flight," he said.

Rappel was handed an 18 month sentence for the more serious assault and six months for the less serious.

The sentence extends his prison term by six months, and his non-parole period by three months.

Justice Elkaim said: " ... having regard to the CCTV footage that I viewed during the trial, it would be wrong to attribute the blame for the whole of the injuries to the two victims to Mr Rappel."

"I can go no further than to conclude that he was a substantial contributor to the injuries."

His lawyers had tried to argue that because Rappel was currently serving his term in Goulburn jail, that any extra time will be more onerous than if he was serving it in the human rights compliant prison in Canberra.

The prosecutor noted Rappel's being housed in Goulburn was not necessarily permanent.

The sentences were partially concurrent with his 32 year term for the murder of Ms Costigan, but Justice Elkaim also extended his non-parole period by three months.

Grech, who is not in custody, was also sentenced on Thursday. The judge handed him a jail term of one month fully suspended on him entering into a good behaviour order for the same period.

Rappel will now be eligible for release on May 26, 2041.