I love the idea behind this piece. The bond between siblings is one of the closest ones I know of, and you represent it well in this piece. The colors flow together well, and I love the way you utilized the ground below them. It shifts with their changing environments. The use of lighter colours in the beginning seems to suggest the brilliance of youth, and as it progresses, the darkness shows the wisdom the years bring on. I love your use of imagination with their clothing and hair styles as they grow older, it's very creative.
Making the last frame of Katara bigger to show her face in relief was a definite plus. I love the shading and the amount of expression you show in her facial features. Overall, it's an amazing piece of art, and I'm glad I stumbled across it in my tumblr-ing.