So, older brother's wife likes the bastard child. Well... when you get married for the wrong reasons sh** like that happens. Step up your game, dude, and woo your wife properly or divorce. I bet he needs her for some other reason like status or family ties. Tough stuff.
Instead of sucking it up it seems like he's pissy with Shultz and his employer. It takes some arrogance to be angry at a boy toy his own family values created. And, it takes some balls to openly threaten another house with a spy. It's pitiful how they project their own failed assumptions at marriage onto Leah and Roxant.
I wonder if, by manga logic, it'll be revealed later that Sebastian had something to do with getting that Prussian maid into the Kroemer house to attack Roxant. He's already shown himself to be an ass about "balance of power" and installing spies. If that's the case and it's found out then he's toast and this obstacle goes away.