When the Spanish writer Gabriela Ybarra heard she had been nominated for the Man Booker International prize, she could hardly believe it. “I had just fed the baby and was going to take a nap,” she says, her four-month-old son gurgling and squawking on her knee, as she speaks at her home in Madrid. “I was super-relaxed. Then I read the email from my UK editor and I couldn’t sleep, because I was so excited.”

The novel that has put this debut author alongside literary stars such as Laurent Binet, Han Kang and László Krasznahorkai is The Dinner Guest, a bold examination of silence and mortality that explores her grandfather’s murder at the hands of Basque separatists in 1977, and her mother’s death from cancer in 2011. The novel was born from Ybarra’s frustration with her mother’s obituaries, and her own struggle to capture the woman she loved in prose.

“I didn’t know how to tell the story of my mother without sounding super-cheesy,” she says, “because all I could think of was good memories.” As she thought about the six months between diagnosis and death, she was struck by her mother’s calm in the face of her imminent end. Ybarra found her acceptance hard to understand. “I thought that death was something completely new to me … When I looked back, I realised that I had lived surrounded by death, but I just couldn’t see it.”

Born into one of Vizcaya’s most illustrious families in 1913, Ybarra’s grandfather Javier fought for the Nationalists in the Spanish civil war and served as mayor of Bilbao from 1963 to 1969. When four Eta militants arrived at his house carrying machine guns in 1977, they handcuffed his children to a bed and bundled him into a car. His body was discovered in a mountain gully a month later.

The family was still under threat when Ybarra was born in 1983, and throughout her childhood – a bomb arrived in the post at their Madrid apartment in 2002 – but nobody ever talked about it.

“Talking about terrorism was completely taboo,” she says. “Although the problem existed and there were bombs exploding every other day, people just pretended nothing was happening.” The silence surrounding the violence in the Basque country extended even to her own family: “In my family there still is a taboo. I didn’t dare ask anybody about [my grandfather] until the book was published.”

Instead of talking to her relatives, Ybarra trawled through newspaper archives for answers that would help her understand her family history – where she came from and who she was. But there were gaps in the media coverage of her grandfather’s assassination, and Ybarra was left with only her imagination to fill them in. “I always tried to be faithful to what I thought would have happened and I tried to make it look as real as possible,” she says, “but many, many passages are completely made up.”

In the second half, where Ybarra turns to her mother’s death, fact and fiction remain difficult to tease apart. “You are always relying on memory and memory tricks you all the time,” she says. “When my sisters and my dad read the book they constantly said ‘This didn’t happen that way, we didn’t go to that place,’ or ‘This wasn’t like that’.”

Readers always want to know how much a work of literature is real, she says, but “the truth is that for me, as a writer, it doesn’t matter. I just want the book to work by itself. The story of the book belongs to the book and real life is something different.”

After ignoring her family’s close acquaintance with death for so long, the act of imagination required to write a novel has made it seem more real. “We live in an equilibrium between fiction and nonfiction, because if we were always aware of all the dangers we have around us, we wouldn’t be able to enjoy anything, we would always be scared,” she says. “Sometimes you have to escape … The problem is that, in my case, I had escaped too much.”

The short, declarative sentences of Natasha Wimmer’s translation reflect the direct, minimalist prose of the Spanish original, a style Ybarra chose partly to allow her some distance: “I was so involved emotionally that in order to understand how it had affected me I needed to take a step back and try to analyse everything as if I were a surgeon.”

Her father was furious when she showed him the manuscript; why, he asked, had she written about something that hadn’t really hurt him? “He had the pain so encapsulated he was convinced he hadn’t suffered,” she says. “After that, he spent two weeks crying.” At first she feared it would wreck their relationship, but instead “it was the opposite. It opened a new channel of communication that had been blocked.”

The Dinner Guest is opening other doors for Ybarra: its Man Booker International longlisting offers her a chance to find readers beyond her Spanish-language audience. “Other countries always look to what is translated into English, so you get more possibilities to get translated into other languages if you are translated into English. [The Booker listing] opens up the world. It’s very exciting.”

• The Dinner Guest by Gabriela Ybarra is published by Vintage.