Isobel is the youngest of the Mauslon girls and Aures's twin. That Isobel is talented cannot be argued: she has a gift with music, and--with a little help from Aures--she has managed to get a relationship from Mhissain no one else has come close to. But despite her being lovers with one of the most powerful women in Nebay, Chloris and Serenka have never considered her as a potential heir to their power, and have always encouraged her to simply help Aures achieve her goals.
It's probably because when she's not trying her hardest to look like a proper Spiran young lady, she's very weird and inappropriate.