When Oyinkan Braithwaite sent an early draft of her debut novel to a few friends, one of them told her it was the best thing she had ever written. “I was offended,” Braithwaite says, her voice heavy with irony. “I knew how I had written it.” She might not have thought much of it at the time, but this quick draft ended up unlocking deals with publishers in the UK and the US, as well as an option from the film company Working Title.

My Sister, the Serial Killer arrived in a feverish month, from a writer in a hurry and never looking back as she poured out a novel in an attempt to break a block. She shakes her head at the memory: “I was a bit mad during that period.”

The block had descended after her short story The Driver was shortlisted for the 2016 Commonwealth Short Story prize. The agent Clare Alexander saw the Nigerian author’s submission and asked her if she had anything else. “That was very exciting for me, but I didn’t have anything that I wanted to give them,” Braithwaite recalls. “So I let a year go by where I kept convincing myself I was going to write something fantastic, and I couldn’t.”

Relaxing on a sofa in her bright yellow jumpsuit, sheepskin boots and green braids, her smile never far from a laugh, it’s hard to imagine Braithwaite paralysed by doubt. It wasn’t that she felt under pressure, she says, but after working for a publisher in Lagos, “you know what’s good and what’s not. I just had high standards for myself.”

As the end of 2017 approached, with her 30th birthday just around the corner, Braithwaite told herself she was being ridiculous: “Forget about the great novel, just write something for yourself that’s fun.”

The novel that broke the dam is a fast-paced, deadpan story of two sisters: nurse Korede and her younger sister Ayoola, a beautiful clothing designer, who has a sizeable following on Instagram and whose boyfriends keep winding up dead. One of them gets food poisoning and starts frothing at the mouth, another starts screaming at her so she stabs him with a knife in what she claims is self-defence. But when Ayoola needs help getting rid of the body, Korede is ready with the bleach.

I stand up and rinse the gloves in the sink. Ayoola is looking at my reflection in the mirror.

“We need to move the body,” I tell her.

“Are you angry at me?”

Perhaps a normal person would be angry, but what I feel now is a pressing need to dispose of the body.

Born in 1988, Braithwaite has a younger brother, Ore, and two younger sisters, Siji and Obafunke. With only two years between her and Obafunke, Braithwaite says the two of them are “going through life together …”

“Sometimes we hate each other. Sometimes we are really not speaking, but I’ve noticed that when push comes to shove we can also band together quite tightly. You know that, whatever happens, that person will be there for you.”

As an older sibling, Braithwaite has an intimate understanding of the responsibility thrust on Korede to look after Ayoola, remembering how her own mother once told her she had to be “like a dustbin. You’ve got to take whatever shit, whatever rubbish comes your way. And you have to be gracious about it.”

A novel that puts the relationship between two sisters at its heart, with men as supporting characters who may or may not make it to the final act, has been greeted as a riposte to crime fiction where the plot is so often set in motion by the gruesome death of young women. “As an individual I’m attracted to strong female characters – my characters have always been people who own themselves. Even if they’re strongly doing something wrong, they’ve always been powerful people.”

As the body count rises and the trauma that shaped Korede and Ayoola’s childhood becomes clear, the novel darkens towards a sensibility that Braithwaite claims for her own. But she says it only goes so far. “I knew it was going to have darkness, but I didn’t want to stay in it, I didn’t want to immerse myself, so it became very matter of fact. I needed my characters to do this thing and then move on.”

This fast pace and light touch was the key to unlocking the humour that fizzes through the novel. “I wasn’t trying to be funny, but my characters and I – we didn’t want to wallow in it.”

Braithwaite spent her childhood in Nigeria and the UK, her family moving to Southgate in north London before she started primary school, then returning to Lagos when her brother was born in 2001. She attended sixth form at a boarding school in Shropshire, then studied law and creative writing at Surrey University and Kingston University before moving back to Lagos in 2012. There she worked as an assistant editor at the independent publisher Kachifo and as a production manager at her father’s education and entertainment company, Ajapaworld.

This back-and-forth upbringing was a long way from the violence of the Biafran war portrayed in Chimamanda Ngozi Adichie’s Half of a Yellow Sun or the political chaos that forms the backdrop to Ayòbámi Adébáyò’s Stay With Me, but Braithwaite is unabashed that her novel makes no attempt to address her country’s difficult past.

“I don’t have that knowledge. I can’t give you what I don’t have,” she says. The idea that writers can represent some universal Nigerian experience is a chimera, when a universal Nigerian experience simply doesn’t exist, she explains. “We have a wide divide between classes and we have a wide divide between cultures because we’re from different tribes, we have different religions. You don’t have to walk very far to see someone who has a really different life from you.” She may live in a house where all you have to do to turn on the light is flick a switch, but in Lagos you only have to cross the road to find someone who has never had electricity. “I wouldn’t want to write a novel and people feel that I’m speaking to a Nigerian experience – I’m speaking to my experience, to the things I’m interested in, and that’s all I can do.”

Thirteen years after the publication of the Kenyan author Binyavanga Wainaina’s satirical essay How to Write About Africa, Braithwaite says there is still pressure for African writers to speak for a whole continent, but that it is beginning to change. “Africans are hungry for different types of stories. Within our own community, people are hungry for something else, for something different. They want more crime, they want more fantasy, they want sci-fi or whatever. Things are already becoming more diverse, but it’s going to become ridiculous – there’s going to be a lot more.”

Nigerian readers may be looking for something a little different, but Braithwaite admits she doesn’t know what they will make of a novel called My Sister, the Serial Killer.

“I’m a little bit worried about that.” A conspiratorial laugh. “I don’t know how they’re going to react. I think some of them will be excited for it, but I think some of them will call me to task, because I’m not speaking to everyone’s experience.” As the head of a drama department at her local church, where she organises plays and sometimes writes scripts, Braithwaite says she was not sure if members of the congregation might object, but, “they haven’t asked me to step down, so,” another laugh, “so far, so good.”

The disquiet about writing such a godless book extends closer to home as well. Braithwaite says she didn’t tell her grandma the whole title of the book at first, referring to it only as “My Sister …” Her parents have already decided it is far too dark for them, and her father asked why she had to write a story without hope. “We would sit down and have these conversations, and he would say: ‘I don’t think they should be putting darkness back into the world. You need to own your responsibility as a creator.’”

Braithwaite hopes she can write something one day that “does justice” to her beliefs, but she is clear that a novelist’s first responsibility is to fiction itself. “I like to have fun,” she says. “The books where I can tell I’m being taught something are a trial in the reading. If there’s a story and you learn something along the way, it’s a bonus.” She still struggles with the tension between her faith and the moral ambiguity of her fiction, but “people say they read it and laughed. I like to think that, in my own little way, I’ve brought joy into the world.”

• My Sister, the Serial Killer by Oyinkan Braithwaite (Atlantic Books, £12.99). To order a copy for £11.43, go to guardianbookshop.com or call 0330 333 6846.