This topic came up recently at Really Bad Eggs with Flambeaux and I thought it might make an interesting discussion in terms of roleplaying games and how one can endeavor to integrate real history and social movements into a game to make it more "realistic" (or less hyper-real, as the case may be!)
Now, entire multipart books have been written on this subject so I am not making any kinds of claims for completeness or a perfect record on accuracy. Nor am I near my personal library this week, so I can't make references or even suggest further reading at this time. Instead, I'm just going to use my best memory as a medievalist to recount to you guys the things I know (or think) about medieval nobility. Anything that is wrong, I will try to correct as soon as I can.
First, before we begin talking about the strategies employed by nobility to preserve familial power and the ways in which they could flex it, it is important to define what exactly the nobility where. There's a distorted perception of them in the public eye, probably because of such psuedo-medieval fantasies of the Victorians and perhaps the things we glean from fairy tales. The view we've been presented of nobility most of our lives (and indeed, of fairy tale environments) is one that is firmly stuck in the pre-modern world. Even the best folklorists cannot trace folk stories earlier than that because one can only go so far back using oral sources.
The nobility of the 1600-1700s is a different thing entirely from the nobility of the middle ages. By the 17th, 18th, an 19th centuries they had become an entrenched class with rigidly defined boundaries. If you were not in the register as a noble you were not a noble. If you couldn't trace your nobility back generations on either side, you were not a noble. There were strict rules defining what nobility meant and who was allowed to be a noble--and there were attendent rights that came along with being noble.
The earlier middle ages is almost the reverse of this: you are a noble because of the rights you enjoy, you don't enjoy rights because you are a noble. Nobility was not a class but rather a quality. You could be possessed of nobilis no matter who your birth parents were. Gentle birth, of course, was preferable, but what constituted it was debatable. This is a much more inclusive system (though systemic factors clearly existed in the favor of established and powerful families) than that of the pre-modern world. If your family or you personally could accrue enough power over your lifetime, you could easily make claim to being gentle (though as the Middle Ages progressed you'd be asked to present evidence of gentility through action as well, which generally means being trained in a wealthy household).
So if nobility where not a rigidly defined class (quite apart from title holding, which is rigidly defined because specific people held titles; but a title was only a manifestation of nobility, not a definition of it) how did powerful noble families act? In what ways could they attempt to ensure the survival of their family? How does any of this apply to your D&D game?
Well, I'm glad you asked! During the 9th and 10th century, continental families of power developed a strategy that involved church holdings. This pattern can easily be duplicated in temple holdings in your D&D world and may take it from swords and sorcery to gritty realism when you realize that those priests your PCs rely so much on are actually engaging in complex dynastic politics. By getting a member of their family appointed abbot or bishop in a region and then holding tightly onto that position, the family could avoid royal taxation (particularly the royal death-taxes) and the poaching of lands by other families by granting them to that monastery or cathedral.
The land would be administered by a powerful family member who ran that particular religious foundation. This would also allow them to accrue command over land that was granted in obitum (after someone died, forever) to these foundations by pious farmers and small landowners. The strategy also caused lands to survive the division of siblings in a central institution (the abbey, the cathedral, in D&D terms the temple, whatever) even when they're hacked apart to give a birthright to each son and daughter's husband.
This strategy was incredibly potent in terms of preserving familial power. Those families that practiced it survived the Carolingian Renaissance stronger than before. Benefices were, at that time, handed out directly from the court and not permanently. The Carolingian kingdom was ostensibly merit-based, with Charles himself picking and choosing who he wanted to serve as his counts (from comitatus, companion) and who he wanted to fill essentially every civic position in all of Francia. This means that titles were not hereditary under Charlemagne.
Ahh, but family power, family power also comes into it. You can buy your positions, or be so entrenched that the king has no option but to declare you count of a certain region, or give him few options, all of them family members. And once Francia was divided amongst the heirs of Charlemagne, all bets were off. As royal power and prerogative eroded, families began to hold on to appointments over many generations, which led to the development of what you might call hereditary titles which were hard for kings to take away.
Just a little bit of history for ya there, and stuff you can integrate into your own game. I find nobility that isn't permanently entrenched to be much more preferable for a medieval style game. For one thing, historical accuracy, but for another it allows the PCs to be comfortably enfolded into the nobility once they reach a certain level or do certain services for the kingdom!