Lisa Dorrian, 25, has not been seen since she attended a party at a caravan park in the Co Down seaside village of Ballyhalbert in 2005.
It is believed she was killed that night or in the early hours of the next morning.
The charity Crimestoppers has put up £20,000 for information leading to the recovery of her body or the conviction of her killers.
The family has also raised more than £60,000 which they say they will make available to anyone who helps find their missing sister.
Detective Superintendent Eamonn Corrigan said: “Despite extensive search operations and thousands of lines of inquiries being completed, Lisa’s body has not been found, and those responsible for her murder have not been brought to justice.
“It’s almost 18 years on, and the pain still felt by Lisa’s family is understandably as raw as ever.
“Lisa’s family and friends won’t ever give up their search for answers and, likewise, our determination remains steadfast.”
Mr Corrigan added: “Those responsible, and those withholding information, must surely have a heavy conscience? They are denying this family the most basic of rights, and that’s the right to lay their loved one to rest.”
