Lindsay Rimer, 13, was found in the Rochdale Canal five months after she disappeared in Hebden Bridge in November 1994.
Despite extensive police inquiries, her killer has never been found.
Kate Rimer said not knowing what happened to her little sister had been a "life sentence" for their family.
Lindsay left her home in Cambridge Street at about 22:00 GMT and visited the Trades Club in Home Street.
The last known sighting of her was caught on CCTV 20 minutes later as she bought cornflakes at a shop in Crown Street.
Her body, which had been weighted down with a stone, was recovered about a mile upstream from the town centre.
Ms Rimer, who was 20 when Lindsay went missing, hopes speaking publicly will help anyone with information to come forward.
"From the beginning we have been filled with grief and we cannot stop the grief and instead, remember her with love until this comes to an end," she said.
"We miss her so much but every memory of her is tainted - it is so painful and bleeds into everything we do.
"It is about time this came to an end; it is a life sentence for this family."
West Yorkshire Police staged a Twitter reconstruction of Lindsay's last movements in November, on the 20th anniversary of her disappearance.
Det Supt Simon Atkinson said they were still looking for a "golden nugget" of information to bring the killer to justice.
The sister of a murdered schoolgirl has spoken publicly for the first time since the discovery of the teenager's body 20 years ago.