With apologies to C.S. Lewis, a modern rendering of The Screwtape Letters 3, Part 4.
Wormwood, you can enter situations that seem most harmless. Have Brother say to Sister, please turn down your music. The words are not offensive. But, in the way they are said, and for the reason they are said, they are not far short of a punch in the face. To keep this game up you and Glubose must see to it that each of these two has a sort of double standard. Brother can insist that the request he made was entirely reasonable and must be judged only on the words spoken. While at the same time, if the roles were reversed, Brother would judge Sister’s words with the most oversensitive interpretation, suspecting that the intention is really to put herself up and himself down. The complaint was registered with the true motive that brother wants the world more to his liking and wants sister’s enjoyment to be lessened.
Wormwood, encourage these two to play this game frequently. Hence, every time, each can go away convinced that s/he is quite innocent, s/he is the victim. Brother can say, “I made a reasonable request and look at the unloving response I received.” Sister can say, “I am innocent and have to put up with statements purely intended for brother to get what he wants. Parents never look past the words to the meaning and so I am always the victim.”
Once this habit is well established you have the delightful situation of Brother and Sister both saying things where they really intend to offend the other, but yet they can claim to be righteous if the other takes offense. If you can succeed in this, Wormwood, you will have won a victory for Satan and created a stronghold for him in the hearts of Brother and Sister.