Before The Ballad of Sexual Dependency, there was Barbara. Nan Goldin's harrowing photos tell the story of her sister's death and how photography saved her

Most of us came to know American photographer and activist Nan Goldin's work through The Ballad of Sexual Dependency, the raw, unflinching 1986 record of her downtown New York circle: the addiction, the violence, the beauty, the loss. Less well known, though, is what made her. Sisters, Saints and Sibyls began not in a Manhattan loft but in a Maryland suburb in 1965, with a teenage girl lying down in front of a train.

That girl was Barbara Holly Goldin, Nan's older sister. Born in 1946, Barbara was a precocious, spirited child who, by her early teens, was being institutionalized, pathologized and controlled by a system (medical, familial, social) that had no language for a girl who simply refused to comply.

She died on 12 April 1965, at 18, by suicide on a railway line near Silver Spring, Maryland. The death certificate reads "from depression". She was on a day pass from a mental hospital.

A few years later, Nan Goldin picked up a camera. "I started to shoot dope, and shoot pictures," she writes in the book. "That saved my life." In eight words, she locates photography not as a career choice or an artistic ambition, but as a lifeline; the thing that pulled her back from the edge her sister had fallen over. Without Barbara, there is arguably no Ballad. Without Barbara, there may be have been no "Nan Goldin, photographer" at all.

Sisters, Saints and Sibyls, first published in 2004 and now republished by Thames & Hudson to coincide with Goldin's retrospective at the Grand Palais in Paris, makes that connection explicit for the first time in book form.

The camera sees what institutions don't

What makes this book extraordinary is its structure. Goldin weaves together three very different kinds of image-making: her father's amateur family snapshots from the 1940s and 50s, her own documentary photographs taken decades later, and the institutional documents (hospital records, psychiatric assessments, a register of orphans) that chart Barbara's official story.

Placed side by side, these sources expose a profound truth about photography's power. The clinical records dehumanise: Barbara is described as having "left her legs unshaven", being "loud and coarse in speech", and as someone who "threatened to become a prostitute". The photographs – of a grinning girl on a tricycle, a teenager watching the sea – tell a different story.

This is the argument at the core of Goldin's practice, and it is one worth sitting with: who controls the image controls the narrative. Barbara's life was documented exhaustively by the institutions that held her, but those documents existed to justify containment, not to understand a person. The photographs resist that. They insist on her humanity.

Goldin began photographing her own circle in the early 1970s, partly because she feared forgetting; feared that without pictures, the people she loved would simply dissolve. That impulse is everywhere in this book. It's why she returned in 2004 to photograph the train tracks, the hospital buildings, the flooded graveyard; places that might otherwise hold no image at all.

For photographers working today, particularly those drawn to documentary or personal work, the lesson is this: the camera is not neutral. Every choice about what to photograph, who to photograph, and how to present those images is an ethical act. Goldin understood that before she had words for it.

In an era when photography is simultaneously more democratised and more disposable than ever, this book is a useful corrective. It is slow, heavy with grief, and deeply serious about what images can and cannot do. It will not teach you anything about exposure or composition. It will remind you why you picked up a camera in the first place.

Sisters, Saints and Sibyls by Nan Goldin is published by Thames & Hudson. Hardback, £30.