In the manual, it states that marriage - given certain conditions (Basically, the same guy or gal is heir to both nations) - you can inherit control of both.
Also, when someone is married off to another faction (champion is lost) the children of that union, when they grow to adulthood, are most likely to join you.
But that's in the manual. (It's in the game folder). It may have been changed in a patch sometime or not implemented yet.