So Gerry Adams knew. And, by his own admission, he knew for years. During a long, rainy walk in Dundalk in 2000, his brother, Liam, confessed to him that he had abused his daughter, Aine.
Gerry Adams was already well aware of Aine's terrible story of childhood rape, had been since 1987, and – again by his own admission – her uncle believed her.
But it wasn't until nine years after that day in Dundalk that the Sinn Fein president finally told the police about his brother's acknowledgement of guilt.
Now it turns out that, in spite of Gerry Adams's claim of a period of estrangement between the pair following Aine's initial revelation, he later attended his brother's wedding and took him canvassing for Sinn Fein in Dundalk.
If he had any internal scruples about this public renewal of association with his paedophile brother, they certainly didn't show.
In 1996, Gerry Adams even sent his niece a signed copy of his autobiography, Before the Dawn, with its foreword thanking all his brothers and sisters – "especially Liam".
It's hard to imagine exactly what combination of hubris, narcissism and selective memory inspired this particular act of avuncular generosity. But Aine's response was visceral. Sickened and upset, she threw the book in the bin.
Now that Liam Adams has been found guilty, Gerry Adams has been confronted with tough questions about his own behaviour, his own prevarications, omissions and excuses.
There is the possibility that he could face prosecution for withholding information from police. As this newspaper has pointed out, "if he was a politician anywhere else in these islands, he would be out of office and party leadership very swiftly. And that is the standard by which we should judge him".
We should, of course we should. But let's face it, we know we won't. We do things differently here. We're backwards like that.
Gerry Adams is likely to survive the repercussions of his brother's guilt for several reasons, both as party president and as TD for Louth.
The obvious one is the fabled loyalty and cohesiveness of the mainstream republican family, an 'ourselves alone' default gut reaction, which tends to kick in sharply in these sorts of situations, automatically over-riding any surface doubts about ongoing political strategies.
Already, his defenders are at work on internet message boards, countering genuine concerns over Adams's potential culpability with the usual combination of paranoia, whataboutery and obfuscation.
And then there's the fact that Gerry is every inch the silverback, the alpha male, gruff and indomitable, replete with uncontested power. Who would be the one to take him down?
Adams is also protected, ironically enough, by unionist cries of condemnation. DUP Health Minister Edwin Poots called on Sinn Fein to explain the difference between the Liam Adams case and the Catholic Church child sex-abuse investigation.
TUV leader Jim Allister demanded that Gerry Adams be prosecuted, arguing that it would show "there is no acceptable level of concealment in these cases."
Both of these are fair points, but the place that they come from renders them inert. Figures like Poots and Allister are seen to be fundamentally motivated by their own tribal loyalties and so their words will inevitably be interpreted in that light. They can't take the high moral ground in a truly disinterested fashion, because they have repeatedly traded that position in the interests of opportunistic political point-scoring.
That's why their criticism can be dismissed as just another jibe. (Though it's true Peter Robinson, in a moment of dignified statesmanship, refused the easy option of going on the attack against Adams and "mak[ing] politics out of a very sad situation". More of this grown-up behaviour, please.)
There are other, even less palatable, factors at work here, too, including a prevailing moral lassitude, or ambivalence about the character of our politicians, which was actually underscored by the Good Friday Agreement.
In adopting that very necessary protocol (there were, and still are, no other workable answers), we also signed up to a new era, in which pragmatism consistently trumps principle.
Ideas of honour, integrity, fitness for high office, are ignored; past atrocities glossed over, made unmentionable.
Is it surprising, in this protective climate, that Gerry Adams's failures, as an uncle, as a leader and as a public figure, should, in the long run, count for very little?