I've found that the game system mirrors what you'd expect in reality perfectly, except for one small thing: it does not have the ability to revert the heir to the firstborn son after his coming of age.

Thus, if a king dies before two levels of heir are in place (ie current heir = future king and 2nd level heir = future heir), the crown will pass out of the direct family, never, it seems, to return

The seeming inability to distinguish between adopted sons and those of the bloodline doesn't help, either.

However, I don't see this in any way as a game-breaking issue; after all, if you've got an especially bad king/heir, there's always the option of going rebel/pirate hunting