I don't understand these worlds. A duke should quite literally have equal, if not, almost as much power as royalty. In terms of military and political power, it should be near equivalent. So I don't understand why princes and kings would commit political suicide by banishing or demoting a duchess. It doesn't work that way in real life. That's basically the King usurping or denying noble privilege (especially since harassment =/= attempted murder or assassination). By doing so, the Royalty is basically starting a cold war with the other nobility, and that's the reason why the Magna Carta existed or how revolutions and civil wars start. In many cases, or almost all cases, the power of all the nobles combined is greater than the Royalty.
Its like Japanese villainess writers don't understand basic noble politics whenever they banish a dukes daughter for harassment and expect it to actually happen.