I like the idea of twins for added playability, but it should be so rare, that it only happens once, if that in an entire campaign.

Here's an idea, since there are going to be prisoners, why not have a building available to build: A JAIL. ie Tower of London, an old castle etc. Then you would not have to sell or trade your prisoners back immediately, you could hold them until the most advantageous moment. Mayhaps the prisoners could 'die off' at a rate close to 10% a turn (Not sure if this is high or low)
If you had a prince or King, this would be a great way to 'mess' with the enemy, as their heir is in your hands. Maybe they announce the son or brother as king and then you ransom their old king back, and that would stir up trouble in their country...would the son or brother step down????

-Just a thought