Gerry Adams has publically called on his brother to hand himself in to police after being accused by his daughter of subjecting her to years of sexual abuse as a child.
Police are currently hunting for Liam Adams, the younger brother of the Sinn Fein president, after he failed to appear in court to face 23 charges of abuse against his first-born child Aine.
Waiving her right to anonymity, she has gone public with the allegations after three decades in a bid for 'closure'.
Her high profile uncle is backing her claims and has called on his brother to hand himself in to face the case against him.
Aine, who is now 36 and uses the surname Tyrell, told an Ulster Television documentary her first recollection of the alleged abuse was when she was four.
"My first memory of it, I know it wasn't the first time it had happened, so I don't know what age I was (when it began), but I think the first time I do remember it happening was when I was four," the mother of two claimed in the Insight programme.
The alleged crimes were committed between 1978 and 1983.
Liam Adams, who had been living in west Belfast, went on the run after failing to show up in court for a preliminary inquiry.
A warrant for his arrest was issued last November.
It is believed he may have fled across the Irish border, and it is understood Irish police are assisting the Police Service of Northern Ireland with the search.
Gerry Adams, who was also interviewed for the documentary, said he has known about Aine's claims for more than 20 years and believed her from the outset.
"I believed Aine from the very first time she spoke to me and I have had no reason to change my mind on that issue," said the West Belfast MP.
He added: "This is a hugely difficult personal matter.
"The only way this can be sorted out now is through the courts. I would appeal to Liam to come forward. He should do the right thing by his daughter."
He said he had no idea where his brother was, but if he did he would tell the police. The senior republican has also provided a statement to the police supporting his niece's case.