In the 1980s, Emma Henderson saw her sister move from the mental hospital she had lived in for 35 years to a small home in the community.
She saw Clare thrive and grow in independence, but the question "What if?" has always haunted her.
What if Clare had had a different kind of life? What if she had had the language to communicate her hopes and desires? And what if she had been born now, when people with similar disabilities face a very different future?
The question proved the inspiration for the Oxford graduate's first novel, weaving together themes of medicine, love and loss set against a backdrop of social change.
Clare was the inspiration for the book's main character, Grace, a severely-disabled child who finds love against the odds in a care home.
"I do have memories of going to visit her," says former teacher Emma, who now lives in London.
"But what I think I've done in the book is not only elaborate them, falsify them if you like, but I think I've done quite a lot of time-slipping with my memories."
'Sense of confusion'
Clare was born in 1946 with various physical and mental handicaps for which she was never given a formal diagnosis.
After being left partly paralysed by polio, she was deemed "ineducable", and at the age of 11 went to live in a mental hospital.
Emma says there was no taboo about talking of her sister, but she grew up with a "sense of confusion" about how to explain Clare to her friends.
"I didn't have any language to describe my sister to the rest of the world," she explains. "There were lots of words used, and I did ask, and I did get answers, but they didn't add up to very much.
"Physically handicapped, mentally defective, was one of the phrases and I did sometimes say that, but the look on my friends' faces, aged 10 or 11, with that rolling off my tongue was just perplexed.
"So it was a problem - for even if there was the will to talk about it, there weren't the words, somehow."
Emma says that once Clare left the children's wards, she was able to work in the hospital, which she seemed to enjoy and found sociable.
"My sister did come home, but not very often, partly just because of the logistics of it in those days.
"And when my parents went to see her, if I went with them, we would, as she got older, increasingly go out with her shopping or for a picnic, that sort of thing."
Guilt and anger
Clare spent 35 years in the hospital, until the winds of change ushered in the era of care in the community.
"She didn't leave the hospital until it was due for closure - so everybody had to leave. It was an enormous problem," says the author.
"Just what to do with all these people, especially the ones who were getting on a bit - finding provision for them was very difficult in the new system.
"The responsibility fell back on the local authorities so my sister was found somewhere to live in Hounslow, which is where she originally came from, and where my mother still lives."
"And of course I can't speak for anyone else, but for her it appeared to be a very happy solution.
"For the last few years of her life, she appeared to blossom."
Emma, 52, has written of her guilt and anger "at the fate that dealt such unfair cards". After Clare's death in 1997, she was compelled to write a novel inventing a story that explored the "What if?" question.
"The book is a tribute to the aspects of my sister's character that I remember with great affection," she says.
Emma Henderson's novel Grace Williams Says It Loud has been shortlisted for the Wellcome Trust Book Prize for literature with a medical theme. The winning book will be announced on the evening of 9 November 2010 at: www.wellcomebookprize.org