Two weeks before Christmas in 2013, Catherine Simpson’s father, then in his 80s, walked into his family’s farmhouse in Lancashire to find his youngest daughter, Tricia, hanging dead from a rope tied to a banister. He cut her down using his work knife, rang for an ambulance, and then sat alone with her body and waited. Tricia, 46, had run the family farm with him, despite her frequent bouts of depression, bipolar disorder and psychosis. Catherine Simpson’s tormented, riveting and bleakly funny memoir analyses her sister’s life to try to find out why she killed herself; in the process it becomes a moving evocation of the muck-spattered realities of modern farm life.

The family had owned New House Farm since 1925. When Catherine and her two sisters were growing up there in the 70s and 80s, the farm was full of local relatives and eccentric friends such as Fred the Dead, a much-loved, if unpleasant-smelling, knacker man. The girls’ childhoods were mostly outdoorsy and idyllic, though less taken up with romping through meadows than staging barnyard versions of Miss World competitions and fashion shows.

In the house, however, it was a different story. Here the Simpsons never conversed, indeed barely spoke at all, intimidated into nervy quietness by a mean-spirited mother who “used silence as a weapon” and put-downs to avoid the obligations to express emotion. “Chatting,” writes Simpson, “was not a concept I was familiar with until I began to visit other families, where I realised, fascinated, that they asked questions and other people answered.”

For much of the book Mrs Simpson, a northern matriarch who might have been created by Alan Bennett for a League of Gentlemen spin-off, is portrayed as so extreme a character as to be almost comic, but there are darker implications.

Simpson is unafraid to plait the pastoral with the darker aspects of country life

As adults, Catherine and her elder sister, Elizabeth, moved away for work, and Tricia, after travelling for a few years, returned to the farm. Her behaviour had been erratic since she was eight, but now she increasingly sought out conflict, accused friends of betrayal and lapsed into self-pity; her parents, though, simply refused to discuss or even acknowledge the problem, even when she began locking them in the house to protect them from imagined predators. Taciturnity, a comic staple of rural memoirs, here becomes toxic and tragic as Tricia’s problems became worse. Her fatal decline and temporary incarcerations in a failing health system make for heart-rending reading.

Simpson, who started the book after her mother died in 2006, tells the story partly by scrutinising the private diaries Tricia kept intermittently throughout her life, reasoning “that it was better to write about Tricia and memorialise her honestly than fall back on the old habit of staying silent”. In the diaries we find Tricia drafting her first suicide note aged about 14, experiencing a psychic “visitation” from a dying cousin, and plaintively wishing “I could be who I want to be”; we also see her taking solace in animals and nature. Tricia dreams of horses and cats, and writes with a roughly poetic turn of phrase about the farm (keeping her dog beside her on a moving quad bike is “like balancing a bag of ferrets on a seesaw”). The seasonal and daily routines seem to have helped her retain some stability; it was after the sale of the farm’s milk cows in 2005, when she became too ill and her father too old, to look after them, that she was prescribed her first anti-psychotic medication.

In a way, the real memorial for Tricia is the compassionate and beadily observed account of the Lancashire landscape. Simpson is unafraid to plait the pastoral with the darker aspects of country life: she does not hide the ugly accoutrements of intensive farming, such as pig farrowing crates, and she shows how flashers and gropers sometimes took advantage of all three sisters when travelling alone in open country. That honesty underwrites what should be an enduring addition to writing about both mental illness and rural England. It would be something of a spoiler to say whether or not she finds a reason for her sister’s suicide, but less so to report that she appears to gradually understand and forgive her parents’ silences. “How simple it seems, the idea of sitting down and talking,” she reflects, “but it was not simple, it was very complicated.” That she resolves to write and “leave behind a lifetime of silence” can only be our gain, and dour rural taciturnity’s loss.

Richard Benson is the author of The Farm: The Story of One Family and the English Countryside.

• When I Had a Little Sister: The Story of a Farming Family Who Never Spoke by Catherine Simpson is published by 4th Estate (£14.99). To order a copy for £13.19 go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min p&p of £1.99