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A 1963 edition of Burke's Peerage - the guidebook to aristocratic families and their lineage - falsely recorded the sisters as having died in 1940 and 1961. But Nerissa actually died in 1986 aged 66 and Katharine passed away aged 87 in 2014.
They spent much of their lives in the Royal Earlswood Hospital in Surrey. Friends said they were well aware of their royal connections and would often curtsey when the Queen was on television.
The day Princess Diana got married, they watched on TV and stroked the screen.
Their accommodation was revealed to the world in 2012, when UK TV broadcaster Channel 4 showed a controversial documentary titled The Queen's Hidden Cousins.
It suggested the royal family had behaved uncaringly towards the sisters - with the monarch said to be "hurt" by the show.
A British reporter once tried to get into the hospital to speak to Katharine Bowes-Lyon, while carrying a birthday bouquet of flowers for her.
There was also outrage around three decades ago when an American TV film company went to the Royal Earlswood Hospital and secretly filmed Katharine inside - with interviewer Geraldo Rivera rigging up a camera in a shoulder bag to film the hidden royal.