Quote Originally Posted by Martok
I've inherited lands a number of times, although it's still a pretty rare occurrence. I myself haven't noticed Caravel's observance that there may be a link between happiness and provinces joining your kingdom; but he could very well be right, since I've never paid particular attention to that aspect. Even if maximizing your peoples' happiness matters, though, I still doubt a player's chances of inheriting land from a defunct faction is greater than 10% or so. Unfortunately, it just doesn't happen that often.

Also, just to clarify: You won't inherit lands by marrying off your princesses--you'll only inherit lands by marrying your sons to princesses of other factions.
I don't get this as much now as I used to, this is because when I started out playing the game I used to keep taxes on normal and never set them to high unless I was desparate for cash. I had since adopted the bad practice of Using the highest taxes I can possibly get away with, and amass a huge treasury. I've continued like this now after years of playing. This is why I never see any faction's old lands joining my faction. Also since the AI taxes to the hilt, you never see wiped out faction's lands joining them either.