Joy Hewer was sexually assaulted and left for dead in her flat in Walthamstow, London, on October 17 1995.
The primary school teacher was stabbed to death.
Penny Barnes, Joy Hewer's younger sister has appealed for anyone with information to come forward as the anniversary of her death comes around.
Ms Barnes, 62, said: "We need to see closure and can't face another 20 years of unanswered questions."
Joy Hewer was 52 at the time of her death and her body was discovered in her bedroom after two fires were purposefully started in her flat.
Detectives are searching for a man they saw on CCTV who entered her flat, in St David's Court, around 10:30 on the night she died.
They describe him as white and now between 50 and 60 years old. At the time he was wearing a light jacket and had short hair.
Officers think anyone that knows the man would still be able to identify him.
Joy's sister, Penny Barnes, said that her sister was "absolutely no harm to anyone" and "would go out of her way to help others".
She said: "We've never forgotten the moment we were told she'd been murdered. It will never make any sense to us.
"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."
Enhanced CCTV of a man seen entering Joy's flats in St David's Court, Wood Street, Walthamstow
The police are also looking for specific people who may know more, including a male that called the fire service from a public phone on Fulbourne Road and a person waiting at the bus stop who he spoke to during the call.
Detective Inspector Susan Stansfield, leading the investigation, said: "We are sure Joy would have been cautious about letting someone enter her flat late at night.
"There were no signs of a forced entry and it may be that she knew the person who entered her flat that night.
"Our forensic teams also found two used coffee cups within the address.
"After two decades we realise a lot of time has passed and someone who may have not spoken out in 1995 may feel they can come forward now, 20 years on.
"Joy's parents have sadly passed away but her brother, sister and nieces and nephews still very much hope to see someone brought to justice.
"We hope that this latest appeal will enable someone with the final piece to the jigsaw to speak out."
