Joy Hewer, who had been primary school teacher, was sexually assaulted and left for dead in her flat in Walthamstow, north-east London, on October 17th 1995.
Detective Inspector Susan Stansfield, leading the investigation, will announce on the BBC's Crimewatch programme that a £20,000 reward will be paid for information leading to the arrest and conviction of the murderer.
Firefighters discovered the 52-year-old's body in her bedroom with multiple stab wounds after two fires were deliberately started in the sixth-floor flat.
The devout Christian's family have appealed for help, her sister Penny Barnes said Ms Hewer was "absolutely no harm to anyone" and "would go out of her way to help others".
She continued: "We need to see closure and can't face another 20 years of unanswered questions.
"She'd enjoyed her work as a teacher, adored her nieces and nephews and was fantastic with children.
"She regularly attended church and often helped at charity events. She lived a quiet and peaceful life which was totally at odds with her last terrifying moments."
The Met Police has previously appealed for information about a man seen on CCTV entering St David's Court at around 10.30pm on the night of the murder.
Her niece Jo McCarthy said that Ms Hewer's parents died without finding out what happened to her: "I just don't think that they had any peace."
