Very nice interplay of the sisters here. It actually suggests the yin/yang symbolism present from their very first appearance together, without actually putting them into that position. I like your artwork quite a bit, which means I am definitely putting you on my Watch list, and hope to see more of your fantastic (in more ways than one, as you have two hooves firmly planted in the roots of folklore/mythology/religion/legend that informs modern fantasy, including Friendship Is Magic, and in fact, your images of the sisters could very easily be turned into angels) ponies.
Can't have one without the other can ya? Beautiful work, as always from a great artist such as yourself. Celestia looks beautiful, and the dress matches her personality as well. The ying and yang of Equestria..