Ariadne - Cate Blanchett
In my mind, Blanchett is the only choice for Elena's aunt, who needs to be so many things throughout the course of the story - loyal, loving, jealous, angry, passionate, heartbroken, penitent, cold, hopeful.... Not only does Blanchett have the range to handle all that and more, but she exudes the kind of poise and power Ariadne would have needed to have at the top of her game. Also, she actually looks like she could be related to Elena and Queen Illiana, which is always a nice touch.
I imagine this would be a voice acting gig, though it's possible the CGI team could use Stewart's features to make the suggestion of the good doctor's face in the surface of the magic mirror. I'd want Stewart for the gig because he could handle both the doctor's humor and more clinical side equally well, along with the caretaker-style warmth he feels for Elena. Also, his voice is rich and varied enough that it can make up for the fact that, as a magical mirror-turned-therapist, the character doesn't have any limbs or really defined facial features to act with.
Another voice role, this time for an undead wraith who also happens to be another old family friend of Elena's. Irons is perfect because he can inject so much disdain and sarcasm into his voice while still keeping it wonderfully cultured, which is just what I'd expect from someone who has hundreds of years of knowledge and experience on everyone he talks to. At the same time, he can also sound gentle, which is important because undead wraiths still know how to care about people. I might modify Irons voice a little to give it more of an echo, though - I'm not entirely certain Braeth has a mouth or vocal chords as we recognize them.