Catherine McCartney has said she is convinced republicans are "intent on riding out" the controversy over the murder of her brother Robert in January.
She made her claim as it emerged that two Sinn Féin election candidates were in Magennis's bar on the night the brawl erupted that led to Mr McCartney's death.
Members of the McCartney family, who today fly out from Dublin for their White House meeting with US president George Bush on St Patrick's Day, have insisted they will persist with their campaign until Robert's killers are convicted.
Before travelling, Ms McCartney said yesterday that despite the continuing protestations of support for the family from Sinn Féin leaders such as Gerry Adams none of the 70 witnesses to the killing had come forward with useful evidence.
At the weekend, Ms Cora Groogan, an unsuccessful Sinn Féin Assembly candidate in the last Assembly elections, confirmed she was in Magennis's on the night of the killing. She said she saw nothing. She made a statement to her solicitor, which was passed on to the Police Ombudsman Nuala O'Loan.
Sinn Féin also confirmed yesterday that Deirdre Hargey, a Sinn Féin candidate in the forthcoming May local elections in south Belfast who spoke at the recent party ardfheis, was also in the bar on the night. She was interviewed by the PSNI on the night of the attack.
Ms Hargey has also given a statement to her solicitor, and asked for it to be passed to Ms O'Loan, according to Sinn Féin.
Ms McCartney said she was unaware of how important the evidence was that these two politicians could produce. But what she had learned so far from the PSNI was that no evidence that could convict the killers had yet been produced.
She suspected republicans were engaging in a pretence of support for the family despite the urgings from the IRA and senior Sinn Féin politicians for people to assist in bringing Robert's killers to justice.
Ms McCartney complained that "mixed signals" were coming from Sinn Féin.
The McCartney sisters and Robert's partner, Bridgeen, fly from Dublin to London today en route for Washington to arrive later tonight.
As well as meeting President Bush, who has shunned a meeting with Mr Adams, they will meet Senators Hillary Clinton and Edward Kennedy, who has also refused to meet Mr Adams.
Ms McCartney said private and family donations were funding the trip. She said the family was making no secret about the fact they were "broke".
