Gerry Adams will not be prosecuted for allegedly withholding information about his paedophile brother.
Even though Northern Ireland's Public Prosecution Service (PPS) was "premature" to conclude there was insufficient evidence against the Sinn Fein president, the public interest was better served by using him as a witness, Northern Ireland's Attorney General has found.
John Larkin QC said: "It should be open to the PPS to make a public interest decision that such persons should be treated as witnesses and not treated as suspects by the PSNI or prosecuted without first having to take all those steps necessary in order to ascertain whether the evidential test is or is not satisfied."
Liam Adams was jailed for 16 years in 2013 for raping and abusing his elder daughter Aine Dahlstrom over a six-year period during the 1970s and 1980s.
The abuse began when Ms Dalhstrom, who has waived her right to anonymity, was just four years old.
In a statement Co Louth TD Mr Adams said his sole interest had been to help his niece.
"This has been a very difficult time for Aine and her family and the wider Adams family," he said.
"I welcome the conclusion of the PPS that I should not be prosecuted for an offence of withholding. I committed no offence."
The Sinn Fein leader testified at his brother's first trial, which collapsed in April 2013 for legal reasons.
He told the court that in 2000, during a walk in the rain in Dundalk, his brother had admitted sexually abusing his daughter, Ms Dalhstrom.
Mr Adams made his first report to police about the allegations in 2007 shortly after his party voted to accept the PSNI but did not tell officers about the confession until 2009, when he made a second statement.
Mr Adams did not give evidence at the second trial for technical reasons which were not disclosed by the PPS.
The Attorney General's review followed a request from Barra McGrory, the Director of Public Prosecutions in 2013, but could not be made public until after an appeal which was lost last month.
In his 48-page document, Mr Larkin said the PPS could have sought further clarification to establish whether Mr Adams knew the full extent of the abuse.
"On the basis of the evidence on file, it appears to me that the PPS decision that the evidential sub-test was not met was perhaps premature," he said.
"There was certainly some evidence on the police file that indicated knowledge on the part of Gerry Adams that the abuse perpetrated against his niece amounted to rape or unlawful carnal knowledge and yet no action was taken to clarify this issue."
The report added: "It is not for me to determine whether Gerry Adams should be further investigated nor whether or not in the light of any further investigation, he should be prosecuted in relation to the offence of withholding information.
"In deciding whether such further steps should be taken, it is important to note that frequently in cases involving historical sexual abuse within families there is some degree of knowledge of the offences on the part of family members and others.
"This case, was perhaps no different in that respect from many others. It is of obvious importance that Gerry Adams should not be treated any differently than any other member of the public in an analogous situation."
The police passed a file to the PPS and recommended no prosecution as there was insufficient evidence to meet the evidential test.
Deputy Director of Public Prosecutions Pamela Atchison also claimed Ms Dahlstrom did not want her uncle prosecuted.
Ms Atchison said: "Clearly, her wish was to draw a line in the sand. Her focus was on Liam Adams."