I know that there are regulations in the RF of Europe to avoid dispersion of the family possessions, which are also national heritage.
It's a pity that this same principle was not applied in the Belgian RF after Queen Astrid and Queen Elisabeth's death. Queen Elisabeth's Cartier tiara should never have been sold.
It happens also in the Spanish RF, not so often, that the King's sisters borrow a family tiara. I remember the case of HRH the Infanta Margarita wearing the Mellerio pearls and diamonds waves tiara during the President of Portugal's state visit to Spain. I have nothing against this, because it is not a custom.
On the contrary, I found excessive when Infanta Cristina wore the Cartier pearls and diamonds tiara, at Princess Victoria of Sweden's wedding, while her mother, Queen Sofia, wore the more discrete Mellerio waves tiara. For this, we all know that Queen Sofia prefers to be very low profile, but even if I would like to see her more bejewelled, I like her a lot.