On the fives tiaras wear bythe Queen, the Princess of Asturias, the Infantas ( except for Elena who wear since her wedding a Marichalar tiara) two belongs totally to the Queen, one by inheritance, the Prussian one wore by Sofia and Letizia for their wedding, one as a gift of Spain peolple for her wedding, the Flower one. The Mellerio one ( or Shells and Pearls) belong to Grand Mother of the King( I don't remember if it's a gift of Alfonso XIII but it's a Cartier's work of the 1920? about) belong to the Spain Royal Family as a "bijoux de passage" i.e. belong to the king for his wife, exactlu as the Fleur de Lys tiaras
The last one, wore recently by the Queen in Norway belong to the sister of the King the Infanta Pilar, but it seems that the King had baught to his sister this tiara too regal for the use that the Infanta Pilar , who has others tiaras smaller, can do it. It's the same for her daughter Simotta Gomez Acebo or her daughters in law.
For the Mellerio tiaria, we can observed that the Infanta Margaritha wore it for the gala diner for the president of Portugal, as a loan of the King.
It's true that with the Princess of Asturias the Spain Royal family should need an other tiara for better turn over! I'm serious.