Netflix's The Crown has a reputation for digging deep into the history of the royals. Fact-checkers usually give the show points for historical accuracy, but events can also be exaggerated or even fictitious.

Season 4 of the drama handled especially controversial topics, particularly those related to Prince Charles and Princess Diana's relationship. But further yet, a storyline in episode 7 on two "hidden cousins" of the Queen caused a major stir. Is it true?

The Queen's cousins: The Crown, "The Hereditary Principle"

In the episode, Princess Margaret (Helena Bonham Carter), confronts The Queen Mother (Marion Bailey) after learning of secretly institutionalized family members: "It's nasty and cold-hearted and cruel and fully corresponds to the ruthlessness I've experienced myself in this family," she says, referring to her cousins Nerissa and Katherine Bowes-Lyon, who spent their lives in an asylum.

Even if The Crown sometimes mixes fact and fiction, these women actually existed. Nerissa and Katherine were born to Elizabeth Bowes-Lyon's brother, John Herbert Bowes-Lyon, in 1919 and 1928. Both sisters had a learning disability and they are said to have not developed according to their age.

Did the Queen's cousins Nerissa and Katherine Bowes-Lyon live in an asylum?

In documentary The Queen's Hidden Cousins (2011), viewers learn that Nerissa and Katherine spent their younger years in the family mansions. It was not until 1941, when Nerissa was 22 and Katherine was 15, that they were taken to a mental institution.

However, it has not been proven whether they ever received a visit from the Royal Family. In the documentary, a nurse said there were no visits, adding: "They never received anything at Christmas either, not a sausage."

The two sisters are said to have been aware of their origins, according to one nurse: "When the Queen or Queen Mum was on TV, they curtsied - very royally, very deeply. Obviously there were some kind of memories. It was so sad." Such a scene was portrayed in The Crown. But Nerissa and Katherine are said to have been "forgotten" by their family over the years.

Queen Elizabeth II stands after signing a visitor's book during her visit to the Defence Science and Technology Laboratory

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Lonely deaths for Nerissa and Katherine Bowes-Lyon

As early as 1961, the sisters were mistakenly declared dead. It was not until 1982 that The Queen Mother was said to have learned of the sisters through a letter from the hospital administration. Her inaction was later treated as a "cover-up" when the British press discovered the story. Nerissa finally died in 1986 at age 66, while her sister Katherine lived on until 2014, passing away at age 86. There was allegedly no big funeral service for either woman.

Helena Bonham Carter in 'The Crown' season 3.

In the Crown episode, Princess Margaret is deeply shocked by how they were treated: "If you're not first in line [...] then you'll be spat out, or you'll be hidden away or worse: Declared dead. Darwin had nothing on you lot — shame on all of you."

The Queen Mum responded by justifying her actions, saying they would've had to deport the cousins, otherwise faith in the lineage of the Crown would have been threatened. 

So, it is fact that both cousins were institutionalized. Whether they were deliberately covered up by the royals or what the family really thought of their fate remains a secret. The Queen has never commented on the lives of Nerissa and Katherine Bowes-Lyon, her first cousins.

See a full fact-check of The Crown season 4 here.

Queen Elizabeth II and Prince Philip Couple Quiz

In 1939, the two met again at a Royal Navy College event where Elizabeth said she fell in love with Philip. How old were they respectively?