Grianznuha has been in exile for so many years; he must have had some children... These children all bear the curse of the "Heart of Winter," and theoretically could take up the mantle of the Siren of the Third.
Ever since the rebellion of the Northern Union, Annan had once suspected... Could it be that those troublemakers were not after seizing power from Dmitry, but instead, sought to rally behind the one who also claimed the title of the Rinfrost Dukedom?
After all, it's unrealistic for anyone outside the "Rinfrost" lineage to become an Archduke because of the old grandmother's reasons. As soon as she wakes up, the usurper would pay the price immediately.
But if the Rinfrost Archduke still carries the Rinfrost surname, then it's simply a family affair—they are merely rallying to one of the branches of the Rinfrost family, not rebelling to seize power. This means that even if they are to be held accountable, they could only be judged by family law, not national law.