I despise liars. Those who mock the love and thrust of others by playing dumb or outright lying are the worst. Not giving the Sword Princess a straight clue is something I can understand. Cheating is what losers call technique after all, but answering your blood-related sister like he did is basically an act worthy of the last circle of hell. She's basically worked it out already, and there's now physical and circumstantial evidence at hand. This guy is literally tormenting her, and the longer he does it, the worst it'll be when she learn the whole of it.
From a narrative perspective, it's also a lazy-as-fuck crutch that too many authors rely on.