The Scottish First Minister is facing calls for discipline
John Mason sparked outrage when questioned whether the killers should be labelled as "murderers" and said he only supported troops "if they do good but not if they do bad".
The Glasgow Shettleston MSP posted the comments on Twitter after being urged to back a campaign to bring the IRA killers to justice.
Dougald McCaughey, 23, and teenage brothers John and Joseph McCaig, aged 17 and 18 respectively, were serving with the Royal Highland Fusiliers when they were murdered.
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They were unarmed when they were lured into a "honey trap" by women who enticed them from a Belfast bar to a lonely spot on the outskirts of Belfast, where they were shot dead in March 1971.
Their families last month launched the Three Scottish Soldiers Campaign for Justice group to try to bring to justice three suspects it claims are living openly in the Republic of Ireland.
Nationalist MSP Jasom Mason suggested IRA terrorist could be considered 'freedom fighters'
He tweeted his comments after he was urged to back a campaign to bring IRA killers to justice
A cousin of one of the murdered soldiers today described Mr Mason’s conduct as an insult and demanded that Ms Sturgeon publicly apologise for her backbencher's behaviour.
It was an insult to my family and also to other people who have lost their lives at the hands of the IRA
David McCaughey also called for the First Minister to take disciplinary action against Mr Mason.
Tories also heaped pressure on the SNP by putting down a motion at Holyrood demanding an apology from the party.
Mr McCaughey said Mr Mason’s tweets were “deplorable” and made him “sick to the stomach”.
He said: “To turn around and say an organisation like the Provisional IRA are freedom fighters – that means you’d need to put ISIS, the Basque separatists and any other terrorist organisation down this freedom fighter route.
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“It was an insult to my family and also to other people who have lost their lives at the hands of the IRA.”
He added: “The man should be ashamed to be Scottish."
The row erupted last week when Mr Mason and Ms Sturgeon were contacted on Twitter and urged to support the campaign but the backbench MSP replied that he was "not taking sides between Irish and British."
When challenged he replied: "You say Irish murderers. Others say Irish freedom fighters. I support Scottish soldiers if they do good but not if they do bad."
A cousin of one of the murdered soldiers demanded Ms Sturgeon publicly apologise
He was also asked if he supported the IRA during the Troubles but replied that he was not taking sides on Irish issues.
He later tweeted: "I have been asked to clarify that my comments were general and not about any specific case. I certainly did not intend to cause any offence."
A spokesman for the Scottish Conservatives said: “As Mr McCaughey says this morning, John Mason’s comments on Twitter were an insult to his family and to the many, many other families who lost a loved-one at the hands of the IRA.“As Mr McCaughey also says, neither Mr Mason nor Nicola Sturgeon has apologised for these offensive comments.
"They should do so in order to reaffirm that we all stand against the terrorists who brought so much tragedy and bloodshed to Northern Ireland.”
Express.co.uk has contacted the SNP for comment.
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