The latest season of Netflix's hit series The Crown delivers on the promise set up by its previous installments, to deliver ample drama and jarring revelations. While Season 4 relies heavily on portraying the tumultuous nature of the late Princess Diana and Prince Charles' marriage as well as then-Prime Minister Margaret Thatcher's impact on the strained atmosphere of Britain, it drops its fair share of shocking bombshells of which one is the fact that Queen Elizabeth II had two first cousins whose existence was kept secret for years. 

The Crown Season 4 begins with focusing on much-publicized aspects of the Royal Family, the assassination of Lord Mountbatten, the visible cracks in Charles and Diana's relationship, the infamous break-in to the Buckingham Palace, etc. But by the seventh episode, "The Queen's Hidden Cousins" delves into the history of the Queen's family to unearth a tragic story, the Queen's "hidden" cousins Nerissa and Katherine Bowes-Lyon. They were the daughters of the Queen's mother, Queen Mother's eldest brother, John Herbert "Jock" Bowes-Lyon and his wife Fenella.

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The episode sees Princess Margaret coming to know about her "sisters" and then, with Queen Elizabeth II, checking the royal record books about Nerissa and Katherine, but discovers that they both have been listed dead. They soon come to know that not only are the sisters still alive but also that cousins of Nerissa and Katherine called Idonea, Rosemary and Ethelreda have also been kept a secret. All because they were born with developmental disabilities and as The Crown puts it, they were never publicly acknowledged so as to save the Royal Family from questions regarding hereditary mental illness. 

Whether it was a mistake or a deliberate decision, both Nerissa and Katherine, were listed as dead in the renowned royal genealogy guide, Burke's Peerage. But as Margaret discovers in the episode, they were still alive, but were kept away from the Royal Family. They had been admitted to the psychiatric ward of Royal Earlswood Hospital in Redhill, Surrey in 1941, when Nerissa was 22 years old and Katherine was merely 15, 11 years after their father's death. Their three more cousins were also living in the same hospital and all five of these rather unknown relatives of the Queen remained under the supervision of professional medical care for the rest of their lives. 

It is in 1996, when the hospital trustees wrote a letter to the Queen Mother in 1982 to notify them about the sisters' existence and the state they were in. This shocking revelation is covered in The Crown Season 4's seventh episode where Princess Margaret confronts her mother upon finding out the life her cousins had been forced to lead. 

"Locked up and neglected. They're your nieces - daughters of your favourite brother. Five members of our close family locked up and neglected!"

All the Queen Mother says in response is to chide her daughter for her naivety and simply explains that they "had no choice."

While there were rumors afloat about the Queen's cousins and the treatment meted out to them, it all gained traction and public attention only in 1987 when a tabloid, The Sun, published a report about the Bowes-Lyon sisters. When the story came out and the Palace was hard-pressed to make a statement, while it confirmed that the Queen was aware of the news story, the Royal Family refused to comment on the scandal, stressing that,|"it is a matter for the Bowes-Lyon family."

A 2011 Channel 4 documentary, The Queen's Hidden Cousins, further investigated the matter and even touched upon Nerissa's medical records that shared while she "makes unintelligible noises all the time," she is "very affectionate... can say a few babyish words."

But while it appeared that the royals had forgotten them, both Nerissa and Katherine remembered their family as shown in The Crown's episode where the two sisters watch the TV in rapt attention when the Queen arrives, and they stand up to salute the national anthem. This is not a dramatized moment as comments of Nurse Onele Braithwaite of Earlswood hospital in the 2011 documentary confirms the same.

"If the Queen or Queen Mum were ever on television, they'd curtsey - very regal, very low. Obviously, there was some sort of memory. It was so sad. Just think of the life they might have had. They were two lovely sisters. They didn't have any speech but they'd point and make noises, and when you knew them, you could understand what they were trying to say. Today they'd probably be given speech therapy and they'd communicate much better. They understood more than you'd think."

The sisters remained away from the Royal Family for the remainder of their lives. Nerissa passed away at the age of 66 on January 22, 1986, and was simply buried without any of the royal fanfare in Redhill, Surrey while Katherine died 28 years later on February 23, 2014, at 87 and was buried alongside her sisters and the other cousins Idonea, Ethelreda and Rosemary.

As per the documentary, only the hospital staff attended their funeral. According to The Telegraph money was donated to the hospital yearly by the royal family but none of its members ever visited the sisters throughout their lives. The Crown Season 4 is now streaming on the official Netflix app.