‘MY nature was…to belong to the savage tribe of brothers.” The words belong to Felix Mendelssohn, writing to his big sister Fanny, another talented composer. He rated her work but didn’t exactly over-promote her case.

As with music, so with literature: there are few books written by brothers about their sisters, fewer still singing their praises. But with Two Sisters, poet Blake Morrison goes some way to correcting this deficiency; here we learn about his sister Gill and his half-sister Josie.

He knows and sighs that we live today in “a culture that treats sibling relationships as inconsequential”. His task is to challenge this assumption and make the lives of Gill and Josie entirely consequential by asking: “Aren’t all lives, however damaged, of importance?”

Asked what he’s currently writing, one friend expresses no surprise: yet another (“more cynically still”) asks if it’s “to complete the trilogy”.

Morrison has previously written with astonishing honesty about both his parents with And When Did You Last See Your Father (1993) and Things My Mother Never Told Me (2002). Both were doctors practising in the rural Trough of Bowland; sister Gill lives next door. Blake has made his escape to the South and success. Both Gill and Blake are long unaware they have a half-sister in Josie who stays nearby. Dad the cad had been having an affair with the wife of the pub landlord.

Dad is controlling, domineering. Gill is chronically unhappy, beaten and bullied: she picks the wrong type of guy and is registered blind in middle age from retinitis pigmentosa. The book could be, in Morrison’s words a “misery memoir, that most despicable of genres” but it’s not.

He’s careful to flesh out Gill, retrieve her happier times. We hear of skiing holidays in Scotland, her joy in sunbathing, and a cheery childhood before her dire experience of boarding school.

But then there’s the drink and the crux of her brother’s quest. Why did Gill become self-destructive?

“If you’re searching for reasons why I turned to drink, you’re barking up the wrong tree,” says Gill, but is she right? Might there be no reasons as such?

“Alcoholics are never short of justifications,” says Morrison. “Someone or something else is always to blame.”

The gap between brother and sister widens. Blake worries about his own guilt thinking he was the more favoured by his parents as the elder, that he had primacy. Gill tells him: “You’ve always been their number one.” He frets that he was neglectful in visiting her and their parents.

Gill and Blake were born in the early 1950s when “sexism ruled … the whole culture worked that way”. A culture that went way back… Morrison neatly surveys some of the better-known sibling pairs in literary and scientific history: the Wordsworths of course, the Herschels, Henry and William James with their sister Alice. Theirs was a world long before International Woman’s Day when sisters had little chance of success: today sisters wouldn’t dream of such subservience. We patiently wait, as Morrison points out, a book called Famous Brothers of Great Women.

Poor Gill drinking herself to death. Poor Blake – whatever he does he’s “on the wrong side, even when I’m on her side”. He admits his writing has a mission: “to demythologise the romance of heavy drinking”.

As such he has little time for the glamourisation of those writers (mainly American) who have hymned the bottle. Addicts, he says, “kill your compassion”. He senses his empathy draining, an encroaching coldness. At the same time his own confessional impulses are unrestrained and he admits he himself drinks “more than I should”.

And then there’s Josie, who Morrison learns for certain is his half-sister a mere eight months before her own death. There’s no escaping the pain and hurt here but he has a way of breaking bad news that’s gently delivered and carefully crafted, despite his nagging worries about “writerly narcissism”. Some truths just have to be told, even if they hurt.

Morrison, like Andrew O’Hagan, tells candid stories that are near unbearable in their brutal honesty. As with Larkin he has a tropism towards sadness. But there’s this too – his is a romantic project, one that seeks the preservation of Gill and Josie’s memory.

And this he achieves quite beautifully.