Basically a boy and a girl (brothers) live happy in a village and they are suddently attacked by another girl who kills their father and destroys the village. After some roundabouts they are captured and taken to a citadel.
The reason the girl wants to capture the boy (not the girl) is because he has some hidden power.
After the ending of the first episode we learn that this girl who is a commander on her father's army understands or realizes that her father is not her real father. She realizes (with the help of other boy and girl) that she is the twin sister of the boy she captures - This is quite easily to understand since both have one eye of each color (red and blue).
They help them escape but are soon recaptured and in the final confrontation the not-father shows his true colours and releases his hidden "wind" power. But in an anti-climatic ending the girl (not the twin) unleashes her true power a…