The army agent, who was given the codename Stakeknife, has been named by the BBC as west Belfast man Fred Scappaticci. He has denied he was an agent.
Northern Ireland's director of public prosecutions wants the new investigation to look at what information the army, MI5 and the Royal Ulster Constabulary's Special Branch received from Stakeknife.
The families welcomed the news but said they did not want the Police Service of Northern Ireland to lead any investigation.
Shauna Moreland's mother Caroline was taken from her west Belfast home and murdered by the IRA in 1994 after being accused of being an informant.
Her killing came just weeks before the IRA ceasefire. She left behind three young children.
Shauna described the announcement as a "step forward".
"I believe if she was an informant her handlers would have been able to step in and save her and if she wasn't an informant, Stakeknife's handlers would have known what was happening," she said.
"There was also a police report that went missing on my mummy. She was reported as a missing person, so where was that investigation?"
Ms Moreland said she wanted to know the truth about what happened to her mother.
She said he wanted "evidence one way or another whether she was or was not an informant and for the people who were involved in her murder to be held accountable".
Frank Mulhern's son Joseph was shot dead by the IRA in 1993 on suspicion of being a Special Branch informant.
Mr Mulhern said he wanted the case to be investigated by an international team of detectives "who would be totally free from any pressure from the security services here".
"My son died a very violent death and he didn't need to die," he said.
"There must have been other RUC informants there who could have said where my son was being held. They may very well have even known he was going to be taken away by the IRA beforehand.
"They must have reported this to their handlers, but the handlers must have done nothing.
"I want justice for my dead son, I want the involvement of the state security forces brought to light and I want the people who covered up my son's and other murders to face the courts."
Some families of IRA murder victims have been giving their reaction to the announcement that the army's most high ranking agent within the IRA is to be investigated about the murder of at least 24 people.