Sinn Fein has urged the sisters of murdered Belfast man, Robert McCartney, to stay out of party politics.
Mr McGuinness said the McCartneys could be manipulated
The party's chief negotiator, Martin McGuinness, told them to "be careful" and not to step over the party political line.
The sisters insist the IRA was involved in the murder with one of them accusing Sinn Fein of taking part in a cover-up.
The family are to fly out to the US on Tuesday to continue their fight for his killers to be brought to justice.
Mr McGuinness said their campaign could leave them open to political manipulation.
One of the McCartney sisters is threatening to challenge Sinn Fein at the polls but Mr McGuinness said they could risk popular support.
"To step over that line, which I think is a very important line, into the world of party politics, can do a huge disservice to their campaign," he said.
"In fact, it can dismay and disillusion an awful lot of people, tens of thousands of people who support them in their just demands."
Meanwhile, it has emerged that two Sinn Fein election candidates and a former party councillor were in the bar on the night of the stabbing.
Sinn Fein confirmed on Monday that a third member, former Belfast councillor Sean Hayes, had given a statement to a solicitor which would be passed to the police ombudsman.
Earlier on Monday, Mr McCartney's sister Catherine called for prospective councillor Deirdre Hargey to give a direct account to the ombudsman.
In a statement to a solicitor, Ms Hargey, 23, who plans to stand in May's election to Belfast City Council, said she saw nothing.
However, Mr McCartney's sister, Catherine, said she should give a direct account to the police ombudsman.
"Giving statements to solicitors is absolute nonsense," she said.
"They should go to the police ombudsman who has skilled investigators. Gerry Adams himself said to do that."
It emerged at the weekend that assembly candidate Cara Groogan, who stood in the Mid Ulster constituency in 2003, was also in the bar on the night of the killing.
Cora Groogan (left) and Deirdre Hargey were in the bar on the night
Ms Groogan also said she did not witness anything in the bar and has given a full statement to her solicitor.
However, Mr McCartney's sister, Paula, said that in her opinion the situation "stinks of a cover-up".
Meanwhile, detectives investigating the murder are planning to interview one of the main suspects in the case.
It is understood the suspect is one of the three men expelled by the IRA after the killing of the 33-year-old.
His solicitor has contacted police and arrangements will be made for the suspect to be interviewed at a later date.
In a statement, police said this was a live investigation into a particularly brutal murder and it was not appropriate to discuss specific issues regarding witnesses or suspects.
Detectives investigating the murder have arrested and questioned 11 men, all of whom were released.
Mr McCartney was stabbed to death after a row in a Belfast bar on 30 January.
The IRA expelled three members over the murder and Sinn Fein subsequently suspended seven of its members.