Assuming both the heir and spare(s) lived to adulthood, what would happen to the spares when the heir assumed their father's role? Would they get a lesser title? Become a commoner but with money? Did what happened to spares vary if they were male or female?
This is pertaining to spares in Europe and Japan (I do know of branch families, but can't find anything on, let's say, the second sons of Daimyos, etc.).
Japanese daimyo had a very practical use for "spares". Japan has an extensive history of adult adoption, in which at almost all levels of society, a head of house could adopt an heir who had all the rights of a biological child. How this worked at the level of the great houses was that a daimyo without an heir would adopt from within the extended network of relations/ allies, as per shogunal approval. So, daimyo family trees are a crazy game of musical chairs to trace. There's an official succession, as often by adoption as birth, but even with adoption, the birth relationships are often very important to follow too.
For example, there are the famous "Takasu brothers": four biological sons of Takasu daimyo, Matsudaira Yoshiatsu.
https://en.m.wikipedia.org/wiki/Matsudaira_Sadaaki#/media/File%3ATakasu_quartet.jpg
The "Four Brothers of Takasu." Left to right: Matsudaira Sadaaki, daimyo of Kuwana domain. Matsudaira Katamori, daimyo of Aizu domain. Tokugawa Mochinaga, daimyo of first Takasu then Owari domain. Tokugawa Yoshikatsu, also daimyo of Owari at another time. Their blood relationship bolstered their working relationship as daimyo of separate domains.
One of the reasons for this constant movement of heirs from one house to another was the horribly high rate of infant and childhood mortality. A branch of the Tokugawa/Matsudaira family with the good fortune to have healthy sons could make up for a less lucky branch who lost all heirs to illness.
