Helping the inquiry into the murder of Belfast man Robert McCartney should be a "litmus test" of Sinn Fein's support for policing, his sister has said.
Robert McCartney was beaten and stabbed to death two years ago
Catherine McCartney was speaking after police issued a new appeal for information on the second anniversary of the killing.
Mr McCartney, 33, was beaten and stabbed to death outside a bar near Belfast city centre.
His sister said Sinn Fein now had no excuse not to assist the police probe.
"If it was purely a policing issue for Sinn Fein that has now been removed," Ms McCartney said.
"Sinn Fein members (in the bar) refused to speak to police. They had a nonsense of talking to third parties like priests, but that was fruitless. Police are trained to take statements, not priests.
"I want Gerry Adams to say he will encourage everybody to come forward and say they will help with the investigation into Robert's murder.
"That will prove there was no culture of cover-up going on."
Mr McCartney, a father-of-two, died the day after he and his friend Brendan Devine were attacked - allegedly by IRA members - inside Magennis' Bar in May Street and then in Cromac Square.
One man has been charged with his murder, and another with the attempted murder of Mr Devine.
However, the McCartney family - who drew international attention with a justice campaign that went all the way to the White House - believe several other people were involved in the attack and others in allegedly mounted a clean-up operation inside the pub to remove any evidence.
Sinn Fein said it had suspended a number of its members after the killing, and Mr Adams previously said he supported the family's campaign for justice.
Meanwhile, PSNI Detective Superintendent Dunwoody appealed for witnesses who had not come forward to now do so.
"I would also address my appeal to those who have declined to speak to police in the past for whatever reason," Ds Dunwoody said.
"I would ask you to reconsider that decision; come talk to me or my team, tell me what you know so that Robert can get justice, his family can get justice and peace of mind and that those who committed this terrible crime can be held to account."
Earlier on Tuesday, Sinn Fein leader Gerry Adams said young republicans had the backing of Sinn Fein to join the PSNI
He was speaking in Dublin after a meeting of the Sinn Fein ard chomairle (executive).
Gerry Adams said he backed republicans joining the police
At a weekend meeting his party voted to back the PSNI.
"If young republicans, or indeed any age of republicans, want to join it that's their right and we would support them doing that," he said.
"There's no point us calling upon people to work with the police if we are not also prepared to support those who want to join, but I think there's a big onus on the PSNI to win that sort of confidence."
DUP leader Ian Paisley said he welcomed any move in support of law and order but still has serious concerns about the republican attitude to policing.
Referring to the murder of Mr McCartney he asked: "What about McCartney?"