My theory with the Kents and Gloucesters and their rents is all down to the abdication.
Henry, Duke of Glouester, was a military man. He was Governor General of Australia and his two sons would have just grown up as grandsons of the King with no real royal role.
Same with the three Kents. They'd have just been the children of the King's cousins.
There may have been a role for the Duke of York, but even his children would be pretty unlikely to have had high royal roles had King Edward VIII played the game as expected and had a suitable wife and children of his own.
Then Henry had to become the regent-in-waiting in case anything happened to George VI before Elizabeth was 18. He couldn't leave England when the King did, just in case, had to leave Australia and take on more royal duties. When the Duke of Kent died even his military career was removed as they couldn't afford to lose him as well with so many young royals and so few adult royals.
The current Duke of Kent was so young when he became so, and he had to step in to fill the gap left by his father (which was mainly created by Edward/David).
So, the Queen treats her cousins much more favourably than anyone would ever expect, or stand for, William treating Beatrice, Eugenie, Louise and co. Because their lives were unexpectedly turned upside down, like hers was, but less dramatic.
I also think that's why Princess Margaret got away with so much as well.
And now that I sound like a complete loon I shall stop!!