document: Joy Hewer, 52, was sexually assaulted and stabbed in her flat in Walthamstow, north east London, in 1995.
  Her younger sister, Penny Barnes, said the family couldn't face "another 20 years of unanswered questions."
  There were no signs of forced entry at the flat and Ms Hewer may have known her killer, police said.
  Her body was found in her bedroom with multiple stab wounds after two fires were deliberately started in her sixth-floor flat.
  An enhanced CCTV image of a man seen entering Ms Hewer's flat in St David's Court at around 22:30 BST on the night she died has been issued.
  He is white, with short hair and wearing a light-coloured jacket. He would now be aged between 50 and 60 years old.
  Officers believe that those close to the man would still be able to recognise him.
  Ms Barnes said her sister was "absolutely no harm to anyone" and "would go out of her way to help others".
  She added: "We've never forgotten the moment we were told she'd been murdered. It will never make any sense to us.
  "She lived a quiet and peaceful life which was totally at odds with her last terrifying moments."
  Police are also searching for a man who called the fire service on the night of Ms Hewer's death from a public phone on Fulbourne Road and a person waiting at the bus stop who he spoke to during the call.
  Lead investigator Det Insp Susan Stansfield urged witnesses who may not previously have come forward to break their silence.
summary: The family of a primary school teacher killed at her home have made a fresh appeal for information, 20 years after her murder.