It seems to me the issue is one of scale. When Sharleyan punished Grand Duke Zebediah, she was acting on both her own and Caylebs behalf as joint rulers of the entire Charisian Empire. When she punished Swayle, she was acting on her own behalf as queen of Chisholm. That left her a lot more wiggle room.
Further, everybody relevant despised Zebediah with a deep and abiding passion matched only by their utter contempt for his stupidity and dishonesty. Swayle, on the other hand, had allies among the Chisholmian peerage to raise a fuss if Sharleyan tried to strip his family of its honors.
