I enjoyed reading your work. It definitely held my interest. A couple of things troubling me were the narration you inserted from time to time and the description of their domicile.
>Their father, who had once been a distinguished member of the Altree Collective, the governing branch of the oldest magical Order of its kind, had long since retired to their little hamlet in the countryside. He’d been burned out by the petty squabbling of his fellow wizards/bureaucrats and left to seek out the relatively peaceful existence of a scholar and family man. It had been many years since he’d set foot in Altree, much to his sons’ dismay.
>the result of a spell gone awry, (this might be okay - maybe I'm being a nitpick since her siblings would know why her hair was red).
This data seemed somewhat out of place. Is there a way to relate this without narration?
Also, these are obviously wizard children living in a wizard's house. I was expecting their surroundings to be a bit more magical. You failed to include a single mystical object in the description of their surroundings.
I hope this helps! You have a good tale going, keep up it up.