Writers invariably look to their personal life for inspiration. Mark Watson has twin sisters, but he is adamant the sibling relationship in his latest novel, The Knot, is entirely the product of his imagination.
''They are a lot younger than me so there can't be any accusations that the book is autobiographical,'' he says.
That is a relief, given Watson's protagonists, Dominic, and his elder sister, Victoria, end up sleeping together. ''I wanted to explore a relationship not seen very often in literature,'' the 32-year-old English author and comedian says.
Watson admits it was no easy feat portraying with sympathy two characters committing one of life's greatest taboos. ''It's one of the few areas of behaviour that can't be forgiven or explained,'' he says. ''It's a challenge to make a likeable novel out of something people are instinctively put off by.''
Since Sophocles penned Oedipus the King and Antigone, writers have usually taken a dim view of sexual relationships between family members as either non-consensual or leading to disaster.
Yet Watson, who is best known in Australia as a stand-up comedian and is also a regular on British television, takes a different approach in his fifth novel.
The Knot opens in a country church, where the narrator, Dominic, a wedding photographer for 35 years, describes a typical English wedding. He also wearily observes: ''I have seen marriage vows broken on the same day they were made, witnessed a jilting at the altar … I don't think there is anything that can happen at a wedding which I haven't seen.''
Speaking with a gravelly voice from Edinburgh, where he has been performing his latest stand-up act, The Information, Watson says he has long toyed with writing about a wedding photographer, a marginal character in someone else's drama.
Yet the drama in Dominic's own life far outweighs the drunken grooms and nubile bridesmaids who populate his working days. The youngest of three children, Dominic lives in awe of his sister, Victoria, but is gently despised by brother, Max. His parents are typical of the interwar generation - hard-working and decent, but lacking emotion.
Watson says he deliberately cast Dominic as an underdog, a likeable if feckless person who is buffeted by events. ''I tried to stack things in Dom's favour so there'd be room to side with him despite everything that goes on,'' he says.
There is nothing graphic in Watson's exploration of Dominic's relationship with his older sister, which evolves over the novel from hero worship to lover, albeit briefly.
Indeed, Watson is at his most circumspect in describing the consummation of their relationship; like an ostrich with its head in the sand, Dominic shuts his eyes and brain and pretends the woman in his bed could be anybody but his sister: ''I clung on to these thoughts and let myself topple over the precipice.''
Watson says his failure to describe the sex act between brother and sister was not intended as a cop-out.
But, he adds: ''I couldn't see a scenario where it wouldn't be cringeworthy and unpleasant to read. I did feel on the whole that if I went into any more detail it would be that little bit too far for readers.''
Watson also deliberately avoids using the word incest. ''The book is about the narrator's attempts to deal with temptation and then attempt to deal with the actual act,'' he says.
But Watson does not allow his characters to avoid the consequences of their actions, although they do not suffer the calamity that befalls Oedipus, Antigone and most other fictional characters who indulge in such a relationship.
Dominic and Victoria are not the only characters hiding secrets in The Knot; no character emerges squeaky-clean by the end of Watson's novel.
In the past, Watson has described his novels as serious and tragicomic, and The Knot certainly fits that description. Watson's comedy shows, many of which he has performed in Australia, have touched on serious topics such as religion, climate change and cyber-fraud.
''I suppose with stand-up you have to go for the instant laughs,'' he says.
''Even if I'm taking on more complex subjects, it's still always about the punchlines.''
The Knot might be laden with characters carrying dark secrets but Watson's life seems to be an open book. He proposed to his wife of six years, Emily Watson Howes, during a 24-hour comedy show at the Edinburgh Festival Fringe in 2004.
''I suppose a lot of my more annoying qualities were more obvious when we got married,'' Watson says. ''Funnily enough, one thing I get pulled up on is that I don't communicate very well.''
As for his sisters, Watson says one of them has read The Knot and ''she really liked it''. ''When she's back we'll have to discuss it in greater detail.''
The Knot by Mark Watson is published by Simon & Schuster, $29.99.