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.

The President says it's unlikely that power companies will switch back to coal, regardless of Mr Trump's plans to boost production.
Mr Trump has also said he wants the US to pull out of the Paris climate agreement.
But President Obama says this would see the US lose its "seat at the table".
The President's views appeared in a policy forum article in the highly regarded research journal, Science.
The editors believe it is the first time that a sitting President has written such a feature.
In the article, the President argues that a "massive scientific record" shows that climate change is "real and cannot be ignored".
Mr Obama also details the reasons he believes the trend towards a low-carbon economy is now "irreversible".
He points to the fact that between 2008 and 2015 the US economy grew by 10% while emissions of CO2 fell by almost the same amount.
Mr Obama says that US businesses have increasingly seen the financial benefits from cutting carbon through greater energy efficiency.
Citing the examples of corporations like General Motors and Alcoa, the President says the US consumed 2.5% less energy in 2015 than in 2008 while the economy was now a tenth bigger.
There would be a huge financial penalty if economies don't reduce their emissions, Mr Obama writes.
If CO2 continues to rise then global temperatures could go up by 4 degrees C by the end of this century, and that could cost the world economy 4% of GDP.
In US terms that would equate to the loss of federal revenue of between $340bn and $690bn every year.
Pointedly, Mr Obama says that 2.2 million Americans now work in jobs connected to energy efficiency - double the 1.1 million that work in fossil fuel production and electricity generation.
Thanks to hydraulic fracturing, gas has emerged as a transformative energy source, now accounting for 33% of US electricity production.
Despite Mr Trump's plans to revive the coal industry by cutting red tape, "it is unlikely that utilities will change course and choose to build coal-fired power plants, which would be more expensive than natural gas plants, regardless of any near-term changes in federal policy."
Even states that supported Donald Trump in the presidential election had moved heavily to renewables. Iowa generated 32% of their electricity in 2015 from wind, up from 8% in 2008.
On the Paris climate agreement, Mr Obama said this was a "fundamental shift in the diplomatic landscape which has already yielded substantial dividends".
Pulling out of the agreement, as Mr Trump has mulled doing, would see the United States lose its seat at the table, and be unable to hold other countries to their commitments, the President wrote. Continued participation in the Paris process, Mr Obama said, would yield great benefit for the American people and the international community.
However, the outgoing president was careful to offer an olive branch of sorts to President-elect Trump. Mr Obama argued that the targets that the US signed up to in the Paris agreement could be achieved in many different ways and "this does not mean that the next Administration has to follow identical domestic policies".
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The family of a primary school teacher killed at her home have made a fresh appeal for information, 20 years after her murder.
[NEXT_CONCEPT]
Renewable energy sources will continue to grow in the US despite the antipathy of the incoming Trump administration, says President Obama.