Katherine Bowes-Lyon (born 4 July 1926) is a niece of Elizabeth Bowes-Lyon and a first cousin of Queen Elizabeth II. Her parents were the Hon. John Herbert Bowes-Lyon (the second son of the 14th Earl of Strathmore and Kinghorn, and brother of the Queen Mother) and the Hon. Fenella Hepburn-Stuart-Forbes-Trefusis. Burke's Peerage, a guidebook to aristocratic lineage, reported that she had died in 1961. When it became apparent that this was not the case, the editor of the book responded, "If that is what members of the family told us, then we would have included it.
She and her elder sister, Nerissa Bowes-Lyon, were born mentally impaired and lived most of their lives in the Royal Earlswood Hospital in Surrey. They were placed there when they were 15 and 22 years of age, respectively. They reportedly had the mental age of six and were largely unable to speak.
When the hospital closed in 1997, 11 years after Nerissa's death in 1986, Katherine Bowes-Lyon was moved to Kentwin House, a care home for the mentally disabled, which was closed after abuse allegations. She was joined there by three of her first cousins, who had been certified mentally disturbed, all daughters of the Hon.