Who is Queen Sia from Sapphique and what is their importance?

Queen Sia is the stepmother to Prince Giles and Caspar. She rules the world that is known as the realm. When the illusion of the realm is sabotaged, the queen is no longer beautiful and in control. She is old and decrepit instead.

Throughout the novel, the queen tries to prove that Finn is the real Prince Giles. Ultimately, though, this is a guise because she is really trying to create a situation where Caspar becomes King instead of Finn. The queen hires an impostor to claim he is the real Prince Giles. The queen also pays Jared to leave the kingdom, so he doesn't tell Claudia and Finn about her plan.