Before I make with the critique, I just want to say I LOVE THIS. But then, I love all your art. So yeah.
I really like that you've focussed on the sisterly bond between Luna and Celestia, bringing together their themes of night and day with a beautiful sunset. Forgive me if I'm projecting here, but to me that sunset represents the end of the fighting between them, and the beginning of a renewed friendship. After all, they do say friendship is magic, don't they.
Although there are a lot of Luna and Celestia pictures out there, this one is different enough in it's overall theme to be original. As always, I love your style of shiny (or as I like to call them, juicy) ponies, and it really fits the theme.
I've said it before, and I'll say it again: I love your style. In this work in particular, though, I especially like the way that the eye is drawn immediately to the characters' eyes, and then outward to the crowns and necklaces, and onwards. I like the way you've used Luna's shorter mane style, her shy smile, and the placement of her ear behind Celestia's, to make her look smaller somehow. Like she's acquiescing to her sister's rule while still maintaining her own role as ruler of the night. The warm, red tones of the sunset add to the feeling of love and friendship in this picture. I do, however, think that their chests are a little too thin, especially Luna's, who seems to curve so sharply that if you followed that line outside of the picture to her waistline, it would be almost nonexistant.
On looking at this picture for the first time, I was struck by the innocence of the poses, and the gorgeous blend of night and day. Although the eye is drawn to the princesses' gaze, it also sweeps left to right, from day to night, with the clever use of shading in the clouds and green hills. The only thing that spoilt it for me was Celestia's horn - it's root seems to be protruding not from the centre of her forehead, but from her eyebrow ridge.
I think I already said, but I really like your use of shading in this. It's probably the main thing that improves this work over the original, as it makes the ponies stand out more from the background, giving the whole thing more depth. It also helps to neutralise the contrast between Celestia's radiance and Luna's darkness, making them easier to view side by side. I like that you removed the sun and moon from the skies behind them; they were distracting from the main focus. Also the addition of the sunset really blends the themes together better. There are still a couple of distractions for me in this work - the birds in the centre are a little too dark, and I keep being drawn back to them, and the dip in the hills on the bottom left messes with the line of Celestia's mane.
All in all, I think this is a great piece of art, and you should have more confidence in your abilities. You're a fantastic artist, and your juicy ponies are among my all time favourites.