Continue negotiating

You smile as your regal wit returns to you. "Don’t you think your offer is a bit difficult to swallow? From where I’m from, if, and I mean if, you manage to sell this crazy idea to your cousin without being beheaded, you will secure influence on two thrones, and would be able to choose which one of my heirs to inherit my throne upon my passing. To me, that seems too unbalanced. You and your family will be getting a lot of benefits, while I will be getting just your loyalty and the privilege of sharing a wife, if you would call that a privilege. And that is before we even include my sister in our deal."

The Archduke finally shows some emotion other than that of confidence as he snaps, "I have to share my wife with you!"

With him losing control, you finally gain the initiative.

You stand up from your chair and say, "Look, I would be more than happy to accept your daughter's hand in marriage for that deal, and I'll even be willing to consider letting you have fun with my sister from time to time. Because I respect you, I’m telling you this, what you proposed, it wouldn’t happen.” You hold up your hand to forestall his protest. “Let me explain. I can promise all of those, but they will remain that, promises. Consider this. You will have the ears of two most powerful men in two kingdoms. Next, you have my sister as your concubine. Lastly, you’re able to choose the next king. My question is this: do you really think you will last long enough to choose the next king?”

At your suggestion, the Archduke narrows his eyes. “You wouldn’t!”

“I don't feel threatened by your plotting. Even I do, which I'm not, there's no need to dirty my hands, my loyal subject. However, the same cannot be said about my other subjects. More than a few, with gold coins in their treasury, would offer rewards to those willing to do the deed of offing you. Or do you think the nobles in my country would be so docile to accept you just like that?”

The Archduke considers your argument and sighs. “No, you’re right. There’s no way the nobles would accept me becoming so influential and having so much power. With the treaty in place, we couldn’t have someone like me around, just like you couldn't let the former queen go, my king."

You smile. “I’m glad your wisdom has allowed you that unique vista; not many people can understand that which is a pity. Now, I find myself a bit puzzled by your insistence on being a King’s regent. You should understand that your role would be a recognition of your status but must remain a ceremonial one. You would appoint the next king, but you must remain neutral or else a new King’s regent will take your place upon your untimely demise. Surely, you don’t want that, do you? So please enlighten me on how this allows you your desired outcome."

The Archduke smiles nervously. “You’re right, my wise King. I neither seek to put the heir from my daughter nor anyone from my family on the throne. It’s just that, I seek the perk that will be accorded to me.”

“A perk?”

“Your harem,” He whimpers in shame at his own lust.

“Ah, I see.” It makes sense to you now. As a king, you can maintain access to a large number of concubines under your harem, someone like the Archduke couldn’t afford to do. So, he basically wants access to them. You finally understand the Archduke’s primary motivation. While he may seek power, wanting to have sex with many women as he could is the Archduke’s real focus. That knowledge is something you can work with. You pretend to think deeply, frowning for a few minutes before you sigh. “Very well. Succeed and you shall have access to it.”

Archduke Wilbur's face lights up with joy on hearing your confirmation, and he bows. “Your loyal subject is thankful for your boon, Your Majesty,” Then he rings a bell. “This calls for a celebration,”

And in walks ten females, all of different races and nationalities.