The witness loosened his tie and unbuttoned his shirt to show the jury how he said his former teacher David Kisun had rubbed his chest when at school in the early 1980s.
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At times, the man said, when the boys had changed into sports clothes, Mr Kisun asked him to stay behind, and then reached into his shorts as he sat on the teacher's desk.
But Mr Kisun, 71, is not on trial over these allegations.
The witness is the brother of a man who on Monday told the ACT Supreme Court the former Marist College teacher had reached into his pants and shirt, and for which Mr Kisun is on trial charged with three counts of indecent assault.
The boy was 9-years-old, and a student at Marist College.
Mr Kisun has pleaded not guilty to all counts.
While the jury on Tuesday was allowed to hear the alleged victim's brother's allegations, Chief Justice Helen Murrell told them they could only use the evidence in a limited way.
She said prosecutors would try to prove the brother's allegations beyond a reasonable doubt, so they could argue Mr Kisun had a tendency to touch the young boys in his class.
If the jury found the brother's allegations proved, it could use that as a piece of circumstantial evidence to be taken into account in considering the allegations being tested at trial.
Giving evidence from a remote witness room on Tuesday, the victim's brother said he had been put in the back row of Mr Kisun's class as one of the better performing students.
He said it was when students were working on individual assignments that Mr Kisun would come to the back of the classroom and undo his button and tie.
He said he had spoken to his brother about what had happened with Mr Kisun while they were both still at school. "It was a talking point in the schoolyard," he said.
The witness told his brother, "that's what he does".
The witness said he also spoke with his brother about what happened when they left school and were in their 20s.
They were sitting on the deck at their parents' Canberra home.
Remember "that bastard Kisun and what he did to us back in school," the witness said his brother told him back then.
He said as his brother recounted the allegations, he had flashbacks and remembered the same thing happened to him.
On cross-examination defence barrister Winston Terracini, SC, questioned the man about his police statement, where he said he first replied to his brother's "bastard" comment with, "what are you talking about?"
He said it had been so long since what had happened he had either forgotten it or put it far back into his mind. "It hadn't been in my brain during that period," he said.
But he said as his brother spoke he began "vividly recalling" his own experience.
The witness also said he was aware of his brother's problems with drinking, illicit drugs and his gambling addiction.
The trial continues.
