The IRA broke the proclamation of the Irish Republic when it shot dead a 15-year-old boy with learning difficulties, his brother said.
Bernard Teggart was in the care of the De La Salle Christian Brothers religious order when he was abducted from St Patrick’s Detention Centre in Belfast in November 1973.
He was wrongly accused of being an informer.
Mr Teggart’s brother John said: “How far towards a united Ireland would they go, shooting a 15-year-old in the head?
“In my eyes they had broken the Proclamation, in my eyes our children will not be all treated the same.
“Bernard Teggart will not be treated the same as any other Irish citizen.”
The 1916 Proclamation refers to “cherishing all of the children of the nation equally”.
Mr Teggart’s protective twin brother Gerard was also abducted, but later released and given money to get a bus home.
He was the first to raise the alarm, but too late.
John Teggart said: “The last thing that Gerard heard were the squeals of Bernard upstairs.
“Gerard has found it very hard to go to sleep.
“He sleeps on the settee. He says it is the ghost on his shoulder.”
Bernard’s body was found at the zoo in North Belfast.
His hands and feet had been bound and a piece of cardboard attached to his shirt labelled him a “tout”.
In 2004 the IRA apologised.
Mr Teggart’s father Daniel died during the 1971 Ballymurphy shootings, which are the subject of ongoing inquest proceedings.
John Teggart addressed a meeting of the Raymond McCord Jnr Forum, named after a victim of loyalists whose father has spent decades campaigning for justice in his memory.
The cross-community forum was debating whether a citizens assembly without political representatives could deal with the past and how it should be made up.