When your little sister is a psycho

I married many men, a ton of them

And yet I was untrue to none of them

Because I bumped off ev’ry one of them

To keep my love alive.

That Rodgers and Hart pop standard, “To Keep My Love Alive,” a jaunty ode to a woman who’s always a widow and never a divorcee, could be the soundtrack to “My Sister, The Serial Killer.” Well, except love’s got nothing to do with it, and men smitten with the fetching and homicidal Ayoola don’t survive long enough to make it down the aisle.

Oyinkan Braithwaite’s rich, dark debut about Korede, a dutiful Nigerian nurse, and her sociopathic younger sister is more nuanced than its title might suggest. Evocative of the murderously eccentric Brewster sisters from the classic play and film “Arsenic and Old Lace,” this fast-paced story feels breezy yet substantial.

As the novel begins, Korede, who narrates the story, is going through a grim task — cleaning up after the murder of Ayoola’s boyfriend, Femi. One minute, the young couple was celebrating their one-month anniversary; the next he was on the bathroom floor, a stab wound in his heart.

“The knife was for her protection. You never knew with men, they wanted what they wanted when they wanted it,” Korede thinks, rationalizing her sister’s latest misdeed. Yet she can’t blot out the fact that this is the third time Ayoola has killed one of her boyfriends.

“Three and they label you a serial killer,” says Korede, who is haunted by Femi though her sister isn’t. Ayoola keeps a comfortable emotional distance between herself and her crimes. A social media junkie, she can’t help but respond when she sees a viral hashtag about Femi’s disappearance. Much to her sister’s annoyance, Ayoola posts a photo of herself with Femi, declaring she was the last person to see him alive.

After Ayoola gets a desperate call from Femi’s mother, Korede tears into her sister. “Do you not realize the gravity of what you have done?” she says. “Are you enjoying this?”

Ayoola is dismayed — but only by her sister’s hectoring. “This is victim shaming, you know,” she says. In her mind, she’s the victim.

Through it all, Korede’s only confidante, Muhtar, lies in a coma at the hospital where she works. She talks to him about Ayoola’s crimes as well as her own unrequited love for Tade, a handsome doctor. Of course, Tade is immediately taken with Ayoola as soon as their eyes meet. If history is any guide, Korede knows a relationship between Tade and her sister won’t end well.

That man may come between the sisters, as one of them tries to come between that man and her sister’s knife. Korede should leave Ayoola to her own devices, but theirs is a bond cemented by the obligations of birth order.

“I am the older sister — I am responsible for Ayoola,” Korede says. “That’s how it has always been. Ayoola would break a glass, and I would receive the blame for giving her the drink. Ayoola would fail a class, and I would be blamed for not coaching her. Ayoola would take an apple and leave the store without paying for it, and I would be blamed for letting her get hungry.”

Braithwaite resists any deep psychological dives into Ayoola’s crimes, but an interweaving storyline about the sisters’ abusive father leaves hints about a possible motivation. Growing up, the sisters knew only fear in the stifling presence of the man they addressed as “Sir.”

“We didn’t call him Daddy. We never had,” Korede recalls. “He was not a daddy, at least not in the way the word ’daddy’ denotes. One could hardly consider him a father. He was the law in our home.”

It is her father’s knife, once his prized possession, which Ayoola used to stab Femi. She’s had the ornate, curved blade since her father’s death; Korede sees the weapon as Ayoola’s accomplice. It’s as if “it were the knife and not her doing the killing,” Korede says. “Who is to say that an object does not come with its own agenda? Or that the collective agenda of its previous owners does not direct its purpose still?”

Though this isn’t a whodunit, “My Sister, The Serial Killer” is a riveting story with a handful of well-timed twists, mixed with laugh-out loud observations about family, co-workers, and corrupt cops.

Braithwaite doesn’t mock the murders as comic fodder, and that’s just one of the unexpected pleasures of her quirky novel. What could have been a series of grisly murders and dead-boyfriend punch lines is instead a clever, affecting examination of siblings bound by a secret with a body count.

MY SISTER, THE SERIAL KILLER

By Oyinkan Braithwaite

Doubleday, 226 pp., $22.95

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