I'm going to make this fairly specific, but it's a situation that will probably be familiar to many people. I'm a king (The Petty King of Ulster, used to be the Earl of Ulster) and after my first wife died I married the Duchess of Normandy. To achieve that I got betrothed to her, and then had her older sister killed. We then had one daughter before she died. Her liege lord is the King of England (one of my allies).
My idea is to marry one of my grandsons (from my first wife) to my daughter, the Duchess of Normandy. Will their son become my vassal rather than the vassal of England? Because this is my goal with the whole thing. I really want to expand my kingdom outside Ireland.
Best Answer
No. A petty king is a duke-level title, thus you cannot have other dukes as vassals.
I am not sure why you think it will work. Your lineage are not automatically your vassals, unless they hold land that are under your de facto rule. Should they gain a title that is on the same level as your own, they will either become independent, or become a vassal of the de facto liege of said title.
The only way I can see you benefit from this, is if you take direct control by marrying her to one of your future heirs. (Ignoring that interbreeding sometimes has funny results).
I assume the duchess (your daughter) is already of your dynasty? Otherwise you might have to convince her to do a normal patrilineal marriage, which she might object to if she is a ruler.
Your succession law can not be gavelkind, and the crown authority of England must be "limited" or "medium", allowing titles to leave the realm by inheritance.
That way, the offspring of the two lovebirds will inherit both the petty kingdom and the duchy. If the Petty Kingdom of Ulster is your primary title, Normandy should then de facto leave England.
Normandy can never be a vassal of Ulster, but you can be a Petty King and a Duke at the same time.