Peerage is how titles are bestowed on to monarchs and ruling classes.
By the time Mitth’raw’nuruodo was alive there were rumors of his family circulating: how the Mitth Ruling Family’s reduced power in the Syndicure. That their new recruits are less capable as other Ruling Families, in ranking and political strength, there was something about Thrawn. He discovers his genetic heritage through his family’s art and possibly who is his family. Then, legal ramifications of his discovery as to strengthen Mitth Ruling Family’s rule and maintaining their genetic bloodlines, while introducing outsiders that can fortify the lines to never go extinction.