There is no rule here because there is no rule about what constitutes the royal family. The Royal house hold has published lists and here is one:
[https://www.royal.uk/sites/default/files/media/annex_d_-_royal_family_11.pdf](https://www.royal.uk/sites/default/files/media/annex_d_-_royal_family_11.pdf)
But the people not on that list aren’t necessarily commoners either.
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