The Public Prosecution Service revealed on Tuesday that it will not be prosecuting over the death of Daniel Hegarty in July 1972.
The 15-year-old was shot twice in the head by a member of an army patrol – ‘Soldier B’ – on duty close to his home in Creggan during Operation Motorman.
Speaking to UTV, his sister Margaret Brady said the family plan to pursue a civil action after hearing of the PPS’s decision.
“We’re not going to give up,” she said.
“We’re going to take a civil action and we will summon him to court – because at the end of the day you can’t get away with murder.”
“It will just send a clear message to all the soldiers that committed murders, shooting innocent civilians, that you can use this excuse that you thought your life was in danger, that’s not right.
“They should be ashamed to even come up with that excuse and they should hang their heads in shame.”
Operation Motorman was an Army attempt to gain control of republican areas in Belfast and Derry that the security forces had considered no-go zones.
After the shooting of Daniel Hegarty – and his 16-year-old cousin Christopher, who was shot by the same soldier but survived – soldiers claimed that the teenagers may have been armed or carrying a bomb.
Decisions not to prosecute were previously taken in 1973 and again in 2008.
An inquest in the 1970s returned an open verdict.
However, the case was re-examined by the Historical Enquiries Team (HET) and they found the RUC's 1972 investigation had been "hopelessly inadequate and dreadful".
That prompted the Attorney General to order a new inquest.
The Hegarty family spoke of their disappointment over the decision.
The file was subsequently referred back to the Director of Public Prosecutions by the coroner to consider if Soldier B should be prosecuted after the inquest found that Daniel Hegarty posed no threat.
On Tuesday the PPS confirmed it would not be pursuing a prosecution.
Assistant Director of Central Casework Michael Agnew said following careful consideration of all the available evidence and information, including the inquest’s findings, that there is no reasonable prospect of a conviction and the test for prosecution is not met.
“Our assessment remains that there is no reasonable prospect of proving to the criminal standard that Soldier B did not act in self-defence having formed a mistaken but honest belief that he was under imminent attack,” he said in a statement.
There have now been calls for the case to be reviewed.
SDLP leader Colm Eastwood said: “This is yet another blow to the family of Daniel Hegarty.
“The Hegarty family deserve justice and this can only prevail when the soldier is brought before the courts and tried for his actions. This case needs urgently reviewed and the PPS need to treat this with the utmost priority.”
Sinn Féin MLA Raymond McCartney said: “I would call on the PPS to review this decision.
"The PPS also need to meet with the Hegarty family to hear their concerns about this decision and I will be contacting the PPS urgently to discuss their decision in this case.”