One of them might be adopted. I think that the brother created his sister with his magic thats why they look alike and the brother use dark magic since its his powers, since she was made using dark magic the darkness forest (that said that any one set foot on it will eventually die if they don't have dark magic and if they do that monster will get them otherwise) since she's made of dark magic shes fine on the forest and that she was made 5 Years ago cuz she doesn't have any memory of it (also the girl from the light kingdom said that she have a dark aura heself you know cuz she was made it!, And they look alike cuz its her brothers magic) Its my theory and i will bet every thing i have taht iam right about it.
WOW I’ll be thinking this as their story for now (▰˘◡˘▰)
the incest bugs me but the plot so far is interesting and shes actively trying to avoid her crazy love filled brother, ill read for now...shes acting like a counselor for him instead and trying to make him be good