Something I've wondered for a while, and then compounded by recent events, is what happens to the relatives of landed nobility following multiple generations of succession. Partly inspired by Crusader Kings, partly by ASoIaF stories, and partly by current real-world events.
If I'm second or third in line to succession, I assume I'd be married off, either for alliances with other realms/nobles, or internally to subjects to stabilise and show favour as needed, or maybe become part of the household? Maybe if I'm a prince, I might be granted a new castle or demesne somewhere, or overlordship of an existing area.
But if I'm a third son, am I ever going to hold land, or even be married off? I have multiple niblings set to inherit before myself, my father owns some land, but the realm is stable, and there's not likely to any free land to grant to me. Is the best I can hope for being a knight in service to my elder brother, or joining a monastery later in life?
There's always more that can be said, but I happen to have a past answer to a very similar question, which I'll post below:
I think fiction gives us this sense that the only important thing to a noble family was who would inherit the title - but in reality, all of their children were important and had parts to play in the continuance of the family. I'm going to look specifically at England in this period, particularly the later medieval era.
The birth order of daughters usually had little bearing on their futures, I should point out. The eldest daughter didn't necessarily have a larger dowry simply because she was the first born, and in fact if a family's standing changed, subsequent daughters could marry higher socially than earlier ones. The first daughter of Elizabeth Hardwick (1527-1608), Frances Cavendish, married an MP - while her youngest daughters, Mary and Elizabeth, married heirs to earldoms, since they grew up with a stepfather who was himself an earl. The only issue with birth order is that it was atypical for an eldest daughter to end up in the church (see next point) rather than an aristocratic marriage.
One important role for a younger son or a daughter in a noble English family was to go into the church - a career that was generally chosen for them by the beginning of their teens. Aristocratic boys would generally be sent to university (Oxford or Cambridge, both of which boomed with new colleges in the fourteenth century) in order to gain an academic religious grounding; it was common for them to be appointed to high-ranking positions in the church with corresponding incomes. Girls, on the other hand, tended to go directly into convents, and might eventually become abbesses. Having a relative in the church was useful in two ways: for one thing, it brought the whole family closer to god, and for another, it gave the family group a foothold in the world of the clergy.
Sons might also be sent to the Inns of Court after university to learn the law, from the fourteenth century on. The Inns were places where lawyers lived and worked, located on Chancery Lane in London, near the Court of Chancery; students came to them as apprentices rather than for more formal schooling, as the Inns are set up for today. This would set them up to have a career in the law or parliament, and as with the church – okay, it’s not the best outcome, but it’d be solid and help the family have another foothold in another social/political realm.
And if none of these were the case - they could always join the gentry. Just because they didn't inherit the main title didn't mean they had nothing! Following thirteenth-century developments that helped fathers move away from the Anglo-Norman standard of strict primogeniture, it was customary for younger sons to inherit more substantial properties. For instance, Thomas Beauchamp (1313-1369), Earl of Warwick, prepared a plan his estates in the event of his death before he went off to war in 1345, leaving more than ten manors in various locations, plus castles etc., to his eldest son, four manors to his second son, and two manors plus other land to his third. Sure, in proportion, the younger sons didn't get very much, but they certainly had enough to live very well, and they would gain more through advantageous marriages to women with nice dowries. As landowners in regions near their inheriting brothers' holdings, they'd form a familial power center together and control significant portions of a county or counties. Kinship networks are very important to understanding how aristocracy functioned.
Many younger sons ended up with titles and extensive lands anyway! The younger son of Henry Percy (Baron Percy), Thomas Percy (1343-1403), was made the first Earl of Worcester by Richard II following his actions in the Hundred Years' War, for instance, and at the end of the middle ages it would become possible to purchase titles. And to go back to the Beauchamps, the surviving younger son ended up inheriting the title of Baron Bergavenny plus twenty-five manors from a cousin who designated him heir. (It's actually way more legalistically complicated than that, but - that's the important bit. If you want to understand the specifics of the legal devices - entails, enfeoffments, and uses - that allowed for creative arrangement of inheritances, try The English Nobility in the Late Middle Ages: the Fourteenth-Century Political Community by Chris Given-Wilson.)