Before the genre became a free-for-all, British crime writers mostly used to obey such informal rules as no unknown poisons, unaccountable intuition, or twins for whom the reader hasn't been prepared.
ell, in fairness to Rev the Hon James Cassells Kyle Paisley, Ian junior's twin and a Free Presbyterian minister from Lowestoft - whose reaction to the recent Irish language row was to describe Gregory Campbell on Twitter as a "liability" who "like those who back him" has "no care for the public face of unionism" - the DUP should have seen him coming.
In March he rebuked those who were stirring up "sectarian strife" by getting worked up about a flag and suggested "a compromise" by which the Cross of St Patrick would fly on non-designated days.
In May he described Peter Robinson as "condescending" and an "ignoramus" for his remarks about Muslims.
Last month, at his father's memorial service, he said his father had brought the DUP and the Free Presbyterian Church to their pinnacle and spoke darkly of fair-weather friends who let you down when you need them.
And now this paragon of consideration and political correctness is once again distressed that Messrs Campbell and Robinson are "shaming unionism".
It may be that Kyle Paisley is bored in his small Suffolk town as he addresses his congregation on subjects like "Should Christians Wear Make-up and Earrings?", and that his only way of getting attention is to exploit his name.
Could it be that - suffused with Christian charity - he's acting out a form of repentance for decades of his father's sectarian bile?
Or is it more likely that, for dynastic reasons, he's getting stuck into a power struggle in a party he isn't even a member of?
Since Ian jnr learned to disown his own earlier homophobic remarks, he's been on the PC wing of the DUP and watches his words. In a Commons debate about libel tourism he even quoted Oscar Wilde - "Who said that the truth is rarely pure and never simple".
You can say that again, Oscar, but here's my guess.
Campbell has some ambition to be deputy leader under Nigel Dodds and is playing the hardman by deliberately appealing to DUPers who love seeing Sinn Fein wound up.
He's being given the nod by that pragmatist Peter Robinson, who is looking over his shoulder at the TUV and Ukip.
So Ian Paisley is hoping to get the throne back with the help of the peace-and-reconciliation wing. Meanwhile, things are hotting up on the Westminster front where everyone's obsessed with what will happen in May if, as seems almost certain, there's a hung parliament.
As the Tories busily woo the DUP, Ivan Lewis, the shadow Secretary of State, is begging the party not to back the Tories, and has set up the Heenan-Anderson Commission to devise policies to help those "left behind by the peace process".
Lewis is no dope, so he also has an eye to Sinn Fein, who would take its seats like a shot if Labour offered a big enough bribe and knows it loves this.
Sinn Fein has spent years organising and attending events in Britain and hanging around Westminster schmoozing those it describes as "progressives", which in an article last week in the Communist Morning Star it listed as "those parties, trade unions and campaigns opposing the economic offensive by right-wing government".
"Our clear view," Conor Murphy MP told a public meeting in parliamentary premises last week, "is that the Agreement is under attack from an anti-Agreement unionist axis". The government "is in danger of sleepwalking into a calamity".
The SDLP punches well above its weight and has some good friends in Labour. Although they're getting on well with the Conservatives these days, unionists are very poor at lobbying. And - as an observer wrote to me - the DUP MPs "hate each other".
Will the DUP - like Sinn Fein - seize the opportunities afforded by the commission to make its economic and political case forcefully?
Or will its MPs leave the goal undefended for nationalists as they rush off to the airport after debates to get the first flight home? If their negligence leads to disaster, guess who the Paisley twins will be blaming.