This is a very, very beautifully done piece of work you have here. I've always liked pictures of Celestia and Luna (as long as they don't involve having them become jerks), be they together, or apart... but especially together. There's just something about having the glory of day and beauty of night side by side that's just so incredibly wonderful, I... I just don't even know what to say! Overall, awesome job. Keep up the fantastic work.
The composition and style is absolutely stunning in this! I love the yin-yang position you did for both sisters and the flow of their hair looks soft enough to touch. The colors are exquisite and the shading is done beautifully. This piece really needs more love and attention for all this work. Keep it up =]