Ah idiots, the bane of civilization. You can't reason with them, they don't understand logic. You can't work with them, they do not understand planning. They will burn the world for short term gains, bite the hand that feeds them. Saddly grandpa needs to make a choice, the morons got the idea in their head that they have ammo, and seem to have become the minions of someone else. They will slip that secret soon. So the choice, silence them, or get ahead of them. Announcing that the heir got married wouldn't be that big of a deal, it just needs to be announced properly before the parents try to make it a scandal. Because really its not at all a scandal, a marriage arranged by the current head. Now if they knew about the love triangles then they would have some ammo, but they do not.
That said the parents do need to be long term dealt with, they will keep circling like vultures till they do damage.