The title of Duchess of Grimaldi was not recreated, but rehabilitated. So, an administrative act. It is the ministry of Justice with the advice of the Diputacion de la Grandeza which takes the decision. The King only signs the RD, like any other RD’s.
Tessa de Baviera, from the spanish branch of the Wittelsbachs, is the widow of Alfonso Marquez (y) Patiño, marquess of Castro.
He was the son of Jose Maria Marquez, marquess of Montefuerte, and Maria del Rosario Patiño, duchess of Grimaldi (1902-42).
A title recreated in favour of his mother by Alfonso XIII, as, apparently, she was a descendant from a Carlos III’s Prime Minister, a Duke of Grimaldi, who died in 1789.
Who was this Maria del Rosario Patiño to earn Alfonso XIII’s goodwill to restore a title that was vacant for more than 100 years ?
A colourful friend of the king ?
Alfonso was himself Consul of Monaco in Spain. Just because of his mother's title ???
Both his parents' titles, Montefuerte and Grimaldi, were inherited by Alfonso’s elder brother, José Joaquin, the 2nd was Count of Paraiso (Count of Paradise cool), the 3rd son, apparently, had no title, just Maestrante de la Real Maestranza de Caballería de Sevilla, and Alfonso, the 4th son was marquess de Castro, a title that either was created to him (may be at his wedding to a bavarian princess) or passed to him sideline since neither of his ancestors used it.