Patricia Da Silva Armani addressed the press conference through an interpreter
Relatives of Jean Charles de Menezes say they are "very disappointed" at the IPCC report into the police's handling of information after he was shot dead.
Mr Menezes's cousin Patricia Da Silva Armani called the report's publication "a very difficult moment for us".
"It's taken two years for this investigation to be concluded," she said through an interpreter.
"It has taken too long for them to get to the conclusion that they have finally arrived."
Mr Menezes, a Brazilian living in London, was shot dead by officers at Stockwell Tube station on 22 July 2005 after being mistaken for a suicide bomber.
The Independent Police Complaints Commission found there were "serious weaknesses" in the Metropolitan Police's handling of information after the shooting.
The Metropolitan Police is to face trial under health and safety legislation in October, but the Crown Prosecution Service decided last year that no individual would be prosecuted in connection with the case.
However, Ms Da Silva Armani told the conference she felt "no-one has been held responsible for anything. No-one is going to be prosecuted".
"The police have been allowed to get away with murder," she added.
"This is a huge injustice and it's shameful."
She said Mr Menezes' family were "very unsatisfied" with the report's findings.
"As the days go by they are trying to get away from their responsibilities that they should be held accountable for," she said.
Family and friends of Mr Menezes campaigned for an inquiry
"I think they treated the life that was taken away by their hands as if it was an animal.
"It's very complicated for us because we know that this person was innocent. So we are very disappointed with everything."
Alessandro Pareira, another cousin of Mr Menezes, said on behalf of the family: "This report shows that the police were a shambolic mess and that senior officers lied.
"They should be held to account. We have waited for two years for this and it is so painful to hear that officers deliberately lied. This is a damning report and it must be acted upon."
Gareth Peirce, the family's solicitor, said the report raised "vitally important issues" that needed to be considered by Parliament.
She said the case showed there had been a "flood" of "erroneous briefings" from the Metropolitan Police.
"There has to be a process of examination to see if intelligence is in fact right," she said.
The IPCC ruled Assistant Commissioner Andy Hayman had "misled" the public.
Liberal Democrat MP Simon Hughes told BBC News the findings meant Mr Hayman's position was "untenable".
"He cannot continue as assistant commissioner of the Metropolitan Police - whatever his previous record and extreme competence.
"He cannot continue with the support and the confidence of the people of London. That's not possible for anybody who reads that report."
However, before the IPCC report's publication, London Mayor Ken Livingstone defended Mr Hayman.
Speaking on BBC Radio 4's Today programme, he dismissed "the idea this is some sort of catastrophic error of judgement on the part of an officer who I have tremendous respect for".
Mr Hayman's "counter-terrorism activity has saved dozens of lives in this city", he added.