The meaninglessness of madness makes it misery.Those among the mentally ill lucky enough to have loved ones cannnot always love in return. Some live in a world of their own, inhabited all too often by unwelcome voices. They may not know what they do, let alone why. If they say "Go away!" what can a father or mother, or, in this case, a sister, do? Mary Loudon, a prize-winning author, has written an account of her search for the trail of her elder sister Catherine, a schizophrenic, who had died in 2001 in a hospital in Bristol without her family knowing she was ill.

Mary had not seen her sister for a dozen years. Catherine left home early in the 1970s. In 1973 their father, a well-to-do general practitioner, was called to India by the Foreign and Commonwealth Office to come and get his daughter, who was wandering deranged and naked.

He managed to bring her from a Himalayan village back to Delhi but she disappeared before he could get her on a plane to Britain. She had, it seems, hidden in the hotel shrubbery while he searched for her.

A year later the family came home one night to find her sitting in the living room. "Hi", she said, beaming at them. "I thought I'd come home."

She did not stay. After spells in prison and mental hospital, Catherine followed a boyfriend to Bristol. They broke up, but she loved Bristol and stayed.

Her illness brought her an assured life tenancy in a council flat. She discouraged her family's attempts to contact her. In January 2001 they learned that she had died – not from suicide, as people would have supposed, knowing her history – but from cancer.

What better quest for a writer than tracing a hidden life? And what better discovery than to find that the lost sister had been living as a man? Catherine had become 'Stevie'.

Loudon traces her detective trail step by step (a bit too slowly and self-dramatizingly, it must be said). She goes to the morgue, to the Bristol Royal Infirmary and to the council flat, where she finds a Tibetan shrine, music tapes and a card on which was written "Please Give Me A Church Funeral".

She talks to the caretaker and various others – the priest, the nun, the doctor, the careworkers – who knew 'Stevie'. She is relieved to find that her sister did have dignity and a established routine. There were people who liked her/him. Each fact unearthed is a treasure: that Catherine smoked a lot, probably the cause of her cancer, that she did her washing regularly, that she enjoyed her Christmas dinner.

Unsurprisingly, the journey turns out to be one of self-discovery. The memories crowd in: Catherine pulling a knife on her small six year old sister and threatening to kill her if she told her parents; the marks on the kitchen wall their mother made to show how tall her children were at different stages. Loudon found the mark for her own height in 1967, but when she looked for the mark from 1972 when Catherine had her most catastrophic breakdown, she found it gone. Had Catherine erased even this trace of herself from the family scene?

Loudon faces directly the predicament of any relative of a schizophrenic. Telling a careworker that she had no photograph of the sister she was trying to trace, he shot back: "What kind of sister are you?"

The guilty sense that one might have tried harder is intensified by the personal reality of a happy marriage and beautiful children. Yet she makes it compellingly clear that there is no way to reach someone who is determined to avoid contact.

There is a moving description of an indomitable mother who is practical, even in her grief, and knows how to accept tragedy: "even in pain, she embraces uncertainty when others might seek explanation".

In passing, but tellingly, the author offers a vivid description of her own post-natal depression, one that required medication for some months.

Most people know that schizophrenia is often controllable with drugs and that organisations such as SANE can give great help. Fewer probably appreciate the loneliness caused by schizophrenia on both sides of the divide between patient and family.

Relative Stranger captures the dilemma poignantly. However, the staccato pace of the book – snappy dialogue, three-word paragraphs, catchy chapter headings such as 'Weird Things That Happen – The Mystery of Frank Sinatra' – leaves an impression of a writer not only in search of a sister, but of a story.