On Friday 5 June 2020, sisters Bibaa Henry and Nicole Smallman threw a socially distanced picnic for Bibaa’s 46th birthday in Fryent Country Park in north-west London. At sunset, Bibaa sent their mother, Mina Smallman, a photograph showing the friends celebrating. Bibaa was a senior social worker and keen cyclist whose own daughter was pregnant at the time. Nicole, 27, a photographer, was artistic, experimental and creative.

The two sisters stayed on in the park after their friends had left, dancing and just hanging out together. They were stabbed to death later that evening in a frenzied attack by an 18-year-old stranger, Danyal Hussein.

From the start, the case wasn’t taken seriously. The two women were reported missing around 5am on Saturday 6 June, but detectives en route to Nicole’s address were told to turn back, and Bibaa wasn’t even logged as a missing person. The police didn’t search the park; it was a group of friends and family who desperately combed the area and found their sunglasses, a knife and then the bodies of Bibaa and Nicole.

Two police officers were assigned to guard the crime scene. Instead, Deniz Jaffer and Jamie Lewis took selfies and posed with the sisters’ bodies, describing them as “dead birds” and sharing the images on two WhatsApp groups with more than 70 colleagues and friends.

A Better Tomorrow: Life Lessons in Hope and Strength is Mina Smallman’s account of the devastation experienced by bereaved friends and relatives after this grotesque violence. It’s also a study of the faults and faultlines of modern-day policing and crime reporting, an examination of institutional racism and misogyny, and a wrenching tribute to two daughters. Furthermore, it is an attempt to transmute “swirling confusion, horror and raw grief” into action. Smallman is propelled by the image of her girls looking down and saying, “‘This is terrible. This has finished Mum!’ No, I couldn’t have that. I couldn’t bring them back, but I could get justice for them… and then for all women and girls everywhere, especially women and girls of colour. It was the only thing that could save me from deepest despair.”

Smallman has thrown herself into public activism, speaking out against racial profiling, misogyny, violence against women and policing practices. Yet at the same time, she torments herself about whether she was a good mother, and searches back through the generations to put herself in context. Tragedy and mental distress haunt the family, particularly on the maternal side: her own mother was a white, working-class Scot born in 1933, whose mother, grandmother and grand-aunt all killed themselves. Her father was a privileged, highly educated Nigerian medical student, so well-dressed that “you could have cut a slice of cake with the seam down the front of his trousers”. The two met on the Manchester jazz scene in the 1950s, but the relationship turns into a tale of disadvantage and poverty, family abuse and mental illness, domestic violence, racism and inequality rolling down the generations and seeping into everything. It’s fascinating, if painful, to see the young Mina develop from neglected child to committed teacher, to Church of England archdeacon and public campaigner.

A Better Tomorrow refuses to wallow in distress or serve up grisly details to the Netflix true crime crowd. Shots of dark humour break through, such as the police officer who searches through tons of discarded rubbish for evidence, acquiring the unfortunate canteen moniker Dave of the Dump. Credit should go to ghostwriter Rebecca Cripps for crafting a crisp account from a traumatic story, although Smallman’s interviews on TV reveal a far livelier voice. I hear that most clearly here in the acknowledgments, where she jokes that: “Because of the range of experiences I’ve had, I described it [crafting the book] as trying to put an octopus in a handbag.” And in her affectionate description of her husband, Chris, “named after Christopher Robin, which is very apt as he loves honey on toast”.

Generally, though, Smallman’s tone remains cool even when describing her nightmares, suicidal devastation, rage, maternal guilt and helplessness, and she pays tribute to the family liaison and victim support workers, murder detectives and others who bring kindness and expertise to the case. At times, however, there is a cathartic release of tension. Faced with a blundering and mealy-mouthed senior officer, she wants to “grab him by the scruff of his neck and throw him into a group of angry mothers who’ve lost their children”.

Women and girls are always told that we are most likely to be abused and murdered, violated and threatened by men we know, even men we once loved or trusted. But in the past few years, Smallman points out, numerous women – Leah Croucher, Sabina Nessa, Ashling Murphy, Zara Aleena, Julia James, Bobbi-Anne McLeod, Ava White and others – have been brutally murdered by strangers. All of these murders were reported and treated differently, and of her own experience Smallman writes: “Two sisters stabbed in a random attack at a picnic. When have you ever heard of that happening before? It should have been a massive story. Everything about it was unusual and terrifying. So the lack of media attention was very concerning.”

She carefully sets out the underlying dynamics. Were her daughters ignored by police officers and newspaper editors because of race? Murdered by a man because of their sex? Stigmatised and stereotyped by powerful institutions because of class? Barely a year later, in March 2021, “we couldn’t help but notice the contrast in reporting levels when Sarah Everard was kidnapped, raped and murdered by a serving Met police officer”.

In the case of Bibaa Henry and Nicole Smallman, the crime is solved, the killer found quickly, and one man who murdered two women has been jailed for 35 years. Yet this does not achieve justice for women overall or end male violence, nor does one case have the power to rectify institutional failings, change the culture or challenge underlying prejudices. Another morning, there it will be again in the headlines. As Smallman writes in this powerful and disturbing book: “What will people think when they look back on this age? They will think we were barbarians.”

• A Better Tomorrow: Life Lessons in Hope and Strength by Mina Smallman is published by Ebury Press (£25). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply