Police say Mr Cronin, 19, was acting as a mediator after a brawl outside a Diamond Creek pub on April 16 when he was hit and suffered fatal head injuries.
There were more than 100 people at the pub at the time and police have more than 100 hours of CCTV footage to canvas.
Brendan Lee sent a mutual friend to one witness’ house on the night of April 19, the same day his brother was charged with murder.
The friend asked the witness to go to a nearby park, where Lee was waiting with three other men, a court heard on Wednesday.
Lee, who had known the witness since the 1990s, made sure he did not have his phone with him.
He said he knew police had been to see the witness, and warned the man’s house could be “bugged”.
Lee told the man he could be charged with affray and suggested he contact a lawyer, who would help retract his statement.
He said: “We all need to stick together for Andy”, the Heidelberg Magistrates Court, sitting in Melbourne, was told.
Lee pleaded guilty on Wednesday to one count each of harassing a witness and possessing explosives, in relation to nine large fireworks found in a search at his Yarrambat home.
His barrister, Philip Dunn, QC, said Lee didn’t tell the witness to lie, and did not threaten him.
“The suggestion is he goes to see a lawyer,” he told the court.
Lee, a construction project manager, lived with his wife and two young children, and had no criminal history.
He was under immense stress at the time he spoke to the witness, Mr Dunn said.
“People sometimes do and say things that, on sober and quiet reflection, they wouldn’t,” he said.
Magistrate Duncan Reynolds handed Lee a $2000 fine for harassing the witness and a $1000 fine for the fireworks, with conviction.
He also said Lee could be released from custody.
Lee had spent about two weeks behind bars after being denied bail on April 21.