There's only one time I can properly recall seeing it happen and, even then, it was one of the AI factions which benefited! I was the English and an already-rampant Spain collected some Novgorod lands through marriage.


If you think about it, for the rules of inheritance (of those times) to work at all, you should have married the dead king's eldest daughter (not the younger, prettier, sister...<g>). Normal etiquette was that younger daughters had to wait their turn anyway (because of the inheritance laws themselves?) but the game doesn't actually enforce that.

It will take close attention, on the part of the player, plus as many emissaries as it takes to have one in every 'court', following faction leaders about (with the attendent risks, of course) in order to spot the appearance of the eldest daughter and to 'chase them down', when they set off, touring the world. Your marriage offer will only work (I should say 'be worthwhile' here) if your heir is still unmarried (as Deus Ret. pointed out, your king never remains unmarried for long enough to catch the princess in time) and could still be turned down anyway, because that faction does not wish to ally with you.

Strangely, the AI will often agree to 'irrelevant' marriages, where it's the king's uncles, or brothers, or the heir's younger siblings who receive the bride and likewise when you send a daughter to them. In these cases, since the prospective husbands are not in direct line to inherit the kingdom, your faction will not inherit the other faction's lands through the marriage.

One remaining puzzle is that, once married, the princess is removed from the game and there's no attempt to continue to track when she dies of old age. In theory though, she should comfortably outlive her heirless father.

What would also be interesting is some proof that you can form an alliance through marriage, then several generations go by and then the other faction dies out. Do you still have rights through marriage from generations earlier, or not? FTSOA, say none of the daughters in subsequent generations had married into other factions and all had married home-faction generals instead?

The point being this:
1st generation daughter from faction A, marries into faction B
3rd generation daughter from A, marries into faction D
4th generation daughter from A, marries into faction F
6th generation king of A dies (no offsping at all). Assume that the aunts/great-(n)-aunts, referred to, above, are also dead by now.

B, D and F all have legitimate claims through marriage but who's to say who has the strongest claim? The one that got there first? The one which was most historically recent? Perhaps the rule, for the purposes of the game, is that if there is the slightest possibility of disputed claims, then nobody inherits?

That might explain why it happens so rarely. But it's just a theory...

One upshot of this might be that, having married one of their daughters, you then have to set about assassinating every other daughter they produce thereafter. Families, eh?