Ahh had forgotten to knock of the old guy so the prince coming of age was not faction heir yet... a small seavoyage chasing rebells let the old king gracefully retire to the bottom of the sea. Then the hunt was on and i finally managed to corner the 3 hearted spanish princess and lay down the proposal for marriage... had to fork out almost 4k in tributes two turns to get her to agree...

Anyways: When you have an unmarried faction heir, then your diplomat get the options to negotiate marriage for the faction heir when engaging a diplomat-princess. Sweet!