Clearing her mother's house, Sara Davidmann found a stash of letters, documents and photographs revealing a closely kept family secret. She tells Hannah Booth about her transgender uncle's hidden life in the buttoned-up 1950s

After Sara Davidmann's mother, Audrey, had been in a nursing home for a year, Sara and her brother began the task of clearing her Oxfordshire house. First they found the "notes" – pieces of paper pinned to clothes and objects as reminders. "Still wearable but only with buttons undone," read one, tacked to a cardigan. Another note attached to a broken watch told of the date it stopped working. There was more: plans of gardens Audrey had dreamed of creating and details of meals people had cooked for her and what she thought of them; and lists of things she needed to buy before she married Sara's father.

Then they found a chest of drawers in the garage. In the top drawer was a copy of every letter Sara had written to Audrey; in the drawer below, two brown manila envelopes. On one was written, in small, neat script: "Ken. To be destroyed." A stash of letters, documents and photographs, it was the full story of a closely kept family secret – one that Sara knew something of, but not the whole story.

Ken was Sara's uncle – he married her "beautiful, glamorous" aunt Hazel – Audrey's younger sister – in 1954. Their wedding photograph shows a gorgeous, dimpled bride smiling in a white dress and veil, and an upright man in a dark suit, with a serious expression. He was 34, she 29.

Among the documents was a letter from Hazel to Audrey, dated 10 September 1959. "This letter will no doubt come as a surprise to you," she wrote, "but don't be unduly alarmed.

"Last October, without any previous inkling whatsoever, I learned that K was changing his sex."

Her husband had secretly been taking female hormones. By July that year his skin was becoming smoother, he didn't need to shave as often and breasts were developing.

Since the age of three, he had wanted to become a girl; as a teenager he collected girl's clothes – even making his own – and dressed as a girl when his parents were out. He constructed false drawers at his parents' – and later his own – house to store them.

In October 1958, four years after they married, Ken decided to tell Hazel. Finding it too hard to say it to her face, he planned to write a letter – but even this proved too difficult. So he resorted to leaving women's clothes lying around for her to "find".

Although shattered by the news, Hazel was extraordinarily supportive, encouraging Ken to dress as a woman at home. "As the strain seemed unbearable as the man, I encouraged K to be the 'woman' whenever he felt that necessary," she continued. But she found it extremely difficult, she wrote: "It wasn't a man dressing up as a woman, it was another person."

Ken's female persona altered the dynamic at home. "K knew I was beginning to resent this woman who was taking her husband and was also taking my place as mistress of my own home," she wrote. "In the role of a woman his personality changes quite a lot. Enjoys doing housework, washing and ironing, fussy about clothes."

Hazel contacted specialists for advice on Ken's hormone treatment, and received a reply from a doctor in December 1958. "Without having seen K this professor said he had 'reached the point of no return' (shattering phrase) … He said it was much too late in K's life to give treatment to go back to masculinity and proposed that K should go for tests and treatment to hasten the progress towards as near femininity as possible."

The enlightened doctor believed it wasn't possible for Ken to resist his tendencies "to wear women's clothes and to adopt female habits and attributes".

But Ken decided not to go ahead with further treatment for Hazel's sake. He would stay as he was – "half man and half woman". Thus outwardly he was a man: he went to work – he was an optician – as a man; he lived in society as a man. But at home with Hazel he was, much of the time, living as a woman.

This continued all their lives: Ken suppressing his desires, Hazel living under the "appalling" strain of it, and both in an apparently unconsummated marriage that was, nonetheless, full of love and companionship. "He clearly loved her so much and didn't want to lose her," says Sara. "But she didn't want him to live as a woman completely."

Ken and Hazel on their wedding day, 1954.

Hazel and Ken kept their secret from everyone, especially his father and physically fragile mother. "K's parents lack understanding of emotional matters, hence K's great fear that they mustn't know," she wrote.

From the cache of documents it appears that Sara's parents visited Ken and Hazel the following year, "probably to see for themselves if it was real", says Sara. Her mother turned sleuth, noting down any evidence of Ken being transgender (Hazel never confessed to him that she'd shared his secret). "Pair of gloves, too large for Hazel," she wrote in one note.

Sara and her siblings knew nothing about their uncle being transgender. Very young at the time of Hazel's discovery, she believes they were kept away from Ken and Hazel's home. "I remember him very little. Perhaps being so young we would have found things we shouldn't have."

Ken died in 1979, aged 59, Hazel in 2003, aged 78. They are, on the wishes of both, buried side by side. Sara's mother passed away in December 2011, and had already inherited Ken and Hazel's store of letters and documents to add to her own, hence the richness of the family archive she left behind.

It has now fallen to Sara, an artist and photographer, to make sense of her family's story. With her exhibition, "Ken. To Be Destroyed", she has photographed and reinterpreted the letters and documents, bringing poignant fragments to the fore and reframing Ken as he always wanted to be: a woman.

She was particularly drawn to the subject matter as, remarkably, without knowing her uncle's secret, she had focused on transgender people in her PhD. "My mother had always resisted my work in this area and now I know why," she says. "Then eight years ago, just before I'd finished my PhD, she sat me down and told me everything. I was gobsmacked."

Does it worry her that she is going against her mother's wishes that these secrets be destroyed? "At the time of my PhD, I told my mother how important it is that transgender people's stories are told – that they are understood as belonging in families, rather than being depicted as isolated, living outside of relationships," she says. "She was happy for Ken's story to be included and she knew I would not be able to keep quiet about it. What I will never know is when she wrote on that envelope – before or after she told me his story."

Her exhibition is finally bringing Ken's true nature out into the open, a place where he would probably have wanted it to be. His story is still relevant today, she believes. Although harder to sustain in more buttoned-up 1950s Britain than in 2013, Ken and Hazel's relationship may not be too different from many transgender people's experience today.

Thanks to her mother's diligence in recording life and keeping letters, Sara's family secret is not just out, but been brought to life. As George Bernard Shaw wrote: "If you cannot get rid of the family skeleton, you may as well make it dance."

Ken. To Be Destroyed is at the Unity Theatre, Liverpool, until 25 November. For help and advice and support on transgender issues, contact The Beaumont Society