Apologies if the topic seems a touch NSFW but I am doing research for a fantasy epic I am writing and lots of its subplots are inspired by historical events and figures, as well as those from different myths, from battles to the people and their motives.
One of the books addresses power dynamics within the royal family and incest within monarchies and how it affects people. So I was wondering if someone could help me in my research. Again, not necessarily inbreeding but just keep siblings around as consorts/concubines/etc.
In the kingdoms of what today is Spain, that was not a thing, although there was one attempt in the 17th century.
King Philip IV sired an unknowable number of bastards, some of which he actually recognised as his children, like Fray Alonso Henríquez de Santo Tomás and Juan José de Austria. In the case of Henríquez, the physical resemblance was so absolutely striking that there was no point denying it. The interesting case is the other guy, Juan José.
Juan José de Austria was an illegitimate son king Philip IV had with the actress María Calderón, better known as La Calderona. The child was acknowledged by Philip, and raised as befitted a royal son, was granted different ranks, knighthood, and even was made a prince, though not actual prince heir. When he was put in charge of the navy, he was created Prince of the Sea so that he would outrank any admiral, even the hereditary ones like the Admiral of Castile and the Admiral of the Indies.
The guy grew ambitious after several military successes, and one day he went to the royal court of his father with a painting. Some say he painted it himself, some say he simply commissioned and presented it. The artwork show him as Jupiter, Margarita of Austria as Juno, and king Philip as Saturn blessing the union.
Philip IV was so completely disgusted and enraged by the idea of Juan José marrying his half sister that he banished Juan José from Madrid forever.
You may find this thread on Ptolemaic sibling marriages, with an answer from u/cleopatra_philopater to be of interest: https://www.reddit.com/r/AskHistorians/comments/7e995d/comment/dq4rfd1/?utm_source=share&utm_medium=web2x&context=3