Heidi Hazell, 26, was shot dead as she sat in a car outside her married quarters in the town of Unna Massen, near Dortmund in West Germany.
Her niece Melanie Anan, who has been campaigning for a fresh inquiry, said she had begun her own investigations.
"I was contacted by a number of witnesses who have come forward and are willing to speak," she said.
"I've always thought that when we have justice for Heidi, this will give us some inner peace and will finally let Heidi rest in peace."
Ms Anan said the initial investigation broke down in 1993, largely due to "sluggish communication" between the British and German authorities.
"You can't conduct a successful investigation if you don't have access to all the witnesses," she told the BBC's Evening Extra.
Ms Anan said her aunt's death had a "devastating effect" on her family.
"Heidi was a civilian and they would have watched her for a period of time before they decided to take her life - they did not make a mistake, they sought her out and murdered her," she said.
Authorities in Germany have reopened their investigation into the 1989 IRA murder of a British soldier's wife.