My mother’s handwriting has always been challenging. I’m used to deciphering it by now, but for her two teenage granddaughters, new correspondence means a fresh, code-breaking session. So it wasn’t unexpected to read recently the advice that an earlier generation gave her: “Your handwriting is really bad. That surprises me, as you are normally so skilful with your hands, you even knit well. Try writing some time with a very soft nib, then you won’t have to press so hard. And make your writing a little bigger. It might be a good idea to use a thicker pen.”

It’s an attentive, but otherwise unremarkable passage. What’s startling, though, is the letter’s context. For this calm, measured instruction was written in late January 1942, at a time when its author, my maternal grandmother, Margarete Oppenheimer, was in mortal danger as a Jew enduring the living nightmare of Nazism, trying to navigate her daily life in the face of insuperable odds. By then, she and her husband, Moritz, had survived seven years trapped in Hitler’s Germany, and 14 months in appalling conditions in French internment camps. What is also remarkable is how the letter – and others that amount to a family history plucked from the destruction of the Holocaust – came to be in my mother’s possession. And it’s all thanks to one very remarkable woman: the family’s housekeeper.

In the very week when senior government officials and SS leaders met in Wannsee to coordinate the logistical task of exterminating millions of other European Jews, my grandmother, by contrast, chose to focus on a solution to the awful handwriting of one of her far-flung offspring. It’s not that she didn’t know what was happening all around her. She knew it far too well. Her rebuke to the suffocating insanity was to keep sending – somehow – thoughtful, loving letters out of the maelstrom. Those few that arrived proved a lifeline to her six frightened children, by then scattered across three continents.

In some respects, my mother, Ruth David, had a fortunate childhood. Her devoted parents were happily married and comfortably off. Her father was the main employer in Fränkisch-Crumbach, the village south of Frankfurt where they grew up. The family even had a housekeeper, Mina.

The housekeeper, Mina. Photograph: Courtesy of Simon Finch

When, just under a year after Ruth was born, my grandmother gave birth again, it was Mina who helped out, looking after Ruth lovingly and spoiling her rotten. The two grew to have a special bond that the other siblings envied. For years, she was like a second mother. But almost all Ruth’s childhood memories are shaped by the fact that she was only four when Hitler came to power in Germany in 1933. As seemingly inconsequential acts of humiliation turned into systematic deprivation, fear became despair. When the order came, via the town crier, that Aryans were no longer allowed to work for Jews, Mina, one of the few Catholics in the village, did her best to ignore it. When my grandfather, Moritz, tried to send her away (difficult, because she had no other home), she refused to go. Eventually, for her own protection, she was forced to leave.

No wonder Ruth retains such vivid memories of her childhood trauma. Eighty years ago, as a nine-year-old, she survived the horrors of Kristallnacht. Her village was such a rural backwater that the destruction came a day late. But it was still ferocious. And Mina was no longer there to protect her. At its end, Ruth’s father and brother were dragged to Buchenwald, the family home left ransacked, and her disabled uncle thrown down the stairs in his wheelchair. Emerging from hiding, she found her mother standing alone in their ruined pantry. Its preserved contents, their lifeline for the winter months, lay smashed on the floor, liquids oozing red through heaps of broken glass.

Half a year later, she was on the Kindertransport, about to begin a new life in England. Her parents had promised they would see her again, but it was a necessary fiction; they died in Auschwitz in August 1942 while Ruth grew up in a refugee hostel in the Lake District under the brutal tutelage of two Viennese matrons. Keeping in touch by letter became increasingly difficult as the war progressed.

Yet remarkably, for the last three years of their lives, some of my grandparents’ letters continued to make it out – intermittently – until only days before they were forced into a cattle truck and transported to Poland to be gassed. Until the start of the war, their correspondence was direct, but soon it had to be sent via third countries, first via a schoolfriend in neutral Holland, then another friend in Lithuania. Both these Jewish acquaintances were murdered once their countries were occupied. Many of the letters never made it through but, looking back, my mother thinks she could not have survived without the ones that did.

After the war, once she learned of her parents’ fate, she wanted nothing more to do with Germany. But, in the late 1950s, two of her English friends persuaded her to return, sensing it was key to her recovery. Their genius was to have tracked down the family’s former housekeeper, Mina, the only German Ruth longed to see again.

My mother barely recognised the old woman, but the two flew together, embraced, hugged tightly and sobbed, rooted to the spot. Mina took my mother back to her little attic bedroom and passed her something she had kept faithfully for years. An old lever-arch file, with the word “Kinder” on the spine, written in my grandfather’s elegant cursive.

It was an archival treasure trove. On the early morning of 22 October 1940, the first major deportation of Jews from the Reich began. My grandparents had one hour to dress quickly, pack a suitcase and assemble at Mannheim station. The plan was secret, yet Mina had heard the news. Not only did she risk arrest by once again meeting my grandparents, she even begged to travel with them into the unknown. Instead, at the last moment, they had thrust an old file filled with letters into her arms and told her to keep it safe until she could hand it over to one of their children. Now, 20 years later, she was making good on her promise.

At first the file’s contents were too painful for my mother to read. It contained all the letters she and her siblings had sent their parents during the war, but also carbon copies of all her parents’ letters to them, up until their first deportation. The ones sent to their elder sons in Argentina and the US, many of which had never arrived, outline their emigration plans and are a detailed account of their early exile.

Inside the binder, her family still lived. The detail in the lost letters allowed her to reconstruct and imagine her parents’ restricted existence. When they wrote to their younger children, they were careful to limit themselves to scraps of reassuring information. But they felt free to write to their older sons more openly, or as much as they dared under the censors’ prying eyes. There was still no mention of the constant house searches of Jewish homes, the hunger, the cold, or the all-pervasive fear. But they did share their struggles: her mother had responsibility for a house full of Jewish orphans, her father at first made unceasing efforts to resuscitate his business, and then to sell it for a price that he hoped would secure him and his wife places on a ship’s passage out.

Once war broke out, they had no resources left. Yet even then there was an occasional triumph. In a hostile land, some of her father’s friendships endured; and one of these meant he secured residential care for his disabled brother and sister in an old people’s home. But it was a temporary reprieve. My mother’s aunt and uncle also died in concentration camps before the war’s end.

Many Kinder believe that their parents gave them life twice, once when they were born, and again when they were sent away to safety. But Mina, too, passed over the gift of life in that lever-arch file. Its contents have allowed my mother to piece together a shattered past, and to pursue her own work on Holocaust education, work that has brought thousands of people face-to-face with the very personal nature of this catastrophe. And it has meant that I can sit with her, now, in the autumn of 2018, trying to decipher the words she wrote to her parents one lifetime ago.