Her sister Clare Williams (born 1946) was placed in an institution in 1957, judged impossible to educate; she was also partly paralysed due to polio.
Clare spent 35 years in hospitals before being released into community care, and died in 1997.
This experience, and the guilt and anger it stirred in Henderson, partly inspired her novel Grace Williams Says It Loud.
Her 2010 novel is about a woman born in 1947 with severe disabilities and epilepsy who is incarcerated in British mental hospitals for thirty years.
[4] The Guardian called it a "sensitive and generous book", although they found the narration was sometimes too detached from the events it described.