When Catherine Loudon died in 2001, the author and journalist Mary Loudon had not seen her sister in over a dozen years. Shocked, feeling an unaccountable sense of loss, Mary began to explore the traces Catherine left behind, trying to discover the truth about her sister – trying to discover who she was. The result of her search is this riveting, moving, and deeply thought-provoking book.

Catherine was always an unsettled personality: when still a child she had threatened her family with violence; when she travelled to India as a young woman, her disturbing behaviour led to her father being summoned to return her to England. She refused to join him, but then reappeared, unannounced, at home a year later. Caring and passionate, but also unstable and paranoid, she was finally diagnosed as schizophrenic. As far as her family knew – Catherine kept in contact with them intermittently, and only on her own terms – her life was a cycle of flats, prisons, and psychiatric hospitals.

Mary Loudon’s quest for her sister begins when she touches Catherine’s cold hands in the harsh calm of a hospital morgue. She visits Catherine’s overpoweringly cluttered flat, and finds herself struggling to choose which of the piled up paintings and clothes she should take to remember Catherine by. Then, over time, Mary tracks down the men and women who inhabited Catherine’s life and the people she affected: the caring nurses who tended to her in her last weeks; the grocer she knew for almost twenty years; the social worker who clashed with her; the minister and nun she prayed with.

Mary Loudon captures each conversation perfectly, with a brilliant ear for spoken language and a telling eye for detail. And though the task seems overwhelming at first, gradually, with each encounter, a more nuanced picture of Catherine emerges. It includes facts that tally with the idolized older sister Mary remembers as well as disturbing revelations, such as Catherine’s self-identification as a man, named Stevie.

In this book Mary Loudon unpicks our preconceived definitions of sanity, belonging, and familial responsibility. Over the course of Mary’s search, we cease to define Catherine by her illness; instead she becomes a human being, full of compassion for the world and possessed of a lively, personal wit. Relative Stranger challenges our most deeply held notions of what makes a life full and valuable – but even though reading it is an education, this is also an undeniably personal and elegiac story, coloured on each page by Catherine’s suffering and the distance that existed between the sisters.

A deeply honest family memoir, a compelling detective story, and a test of our prejudices, Relative Stranger is both a vitally important book and an unforgettable one.