In The Crown, Princess Margaret is appalled to discover that the sisters are alive. She confronts the Queen Mother, who states that the women were placed in the institution amid fears that evidence of mental instability in the Royal Family could threaten the security of their claim to the throne. She says in the episode, ‘Their illness, their idiocy and imbecility, would make people question the integrity of the bloodline. Can you imagine the headlines if it were to get out?’ She goes on: ‘The hereditary principle already hangs by such a precarious thread… Throw in mental illness, and it’s over.’
It’s worth noting, however, that this conversation is completely imagined by the series’ creators. It’s thought that the Queen Mother was in fact unaware of Nerissa and Katherine’s situation until 1982, when – upon learning where they were – she is said to have sent the sisters a check to pay for sweets. Moreover, because the women were committed almost five years after the abdication of Edward VIII, it seems unlikely that the decision was taken in reaction to the family’s close new ties to the throne.