Vassal inheritance warning - Who must die?

I have a duke who has a married a foreign duke which has given me an inheritance warning. This being Crusader Kings, my gut instinct is to have someone killed but I'm not sure who or in what order people must die.

The breakdown is thus: I am playing a Byzantine emperor with high crown authority (shouldn't that prevent titles from leaving the realm?) with a male duke of Jazira reporting directly to me. He has married (non-matralinially) the duchess of Orkney who is a direct vassal of the King of Norge. They have two male children together.

My first thought was to kill my own vassal duke so that his son inherits his duchy and the son becomes my vassal. Then when his mother dies he inherits Orkney which becomes part of my empire since he's already my vassal (at least I think that's what will happen). That's when I noticed a final confusion in the matter, the duchess and duke have different heirs. Their elder son is the heir to the duchy of Orkney while their younger son is the heir to the duchy of Jazira.

I am guessing this has something to do with the differing inheritance laws of the two duchies, the duchy of Jazira has primogeniture succession while Orkney has gavelkind.

All I want is to keep the duchy of Jazira under my control, I don't want Orkney, I have enough problems as it is without it. Who should die to make sure this happens? I suppose I'd accept non-murderous solutions as well, there's a first time for everything.

You kill your own Duke. So long as the Duke's son (whichever) inherits his father's lands before his mother's there is no way you lose Jazira. The chance of the Duchess dying first is why you are getting the inheritance warnings. If she dies first there's a chance that her heir could end up inheriting Jazira as well when the Duke dies. With Orkney having Gavelkind succession, if she has enough holdings there's a chance both of her sons could be landed vassals of another realm when your Duke dies. If both sons are vassals of another country when the Duke dies, you'll lose Jazira for sure.

As Affine and Studoku point out, the reason your High Crown Authority isn't preventing this is because Jazira is not De Jure part of Byzantium. It only can prevent out-of-realm inheritance in the De Jure Empire. I don't understand why Primogeniture succession in Jazira would be giving the younger son Jazira while the older gets Orkney, that's just backwards. However, so long as the Duke dies before the Duchess, and before either son is landed, I can assure you the inheriting son (whoever that may be) will be your vassal.

I know you have said you don't care to have Orkney, but if you did, the solution would be to kill the Duke, then immediately begin plotting to kill the non-inheriting son. This would assure that Orkney went to the son that just became your Duke.

As you say, this is Crusader Kings. All things are possible through adequate murder.

If for some reason you wanted to achieve your goal without killing anyone (a strange concept, but bear with me): If you have a title to spare (even a barony will do) you can land the son that is the heir to Jazira. By giving him the barony, he becomes your vassal. When he inherits further titles those will also become a part of your realm, even if the titles he inherits from outside the realm are of higher rank than his first title. So long as he is landed in your realm, any lands he inherits become and/or remain a part of your realm.