It's easy enough for people to believe this is Molestia kissing Luna, but keep in mind that in the opening story of the pilot, Celestia is shown with a full head of pink hair. This pic is apparently set before the events of Nightmare Moon and Luna's millennium-long banishment.
With that out of the way, I love the completely shocked look on Luna's face. It seems like the sisters were simply hugging when Celestia kissed her out of nowhere, and she clearly didn't expect it or know how to react to it. The pic also takes on an interesting real-life meaning, as incest was practiced between members of royalty in the olden days. Clearly though, this is an interesting take on the situation.
I like how the pic doesn't show the actual kiss; instead it leaves the actual act to the imagination, as all we see is the immediate aftermath.
Leaving the subject matter entirely aside (as people will be reacting to it far more on an emotional basis than an objective basis, and nobody's mind is going to be changed in any case), let's stick entirely to artistic and technical matters. It's safer that way.
What follows are nitpicks. Overall, this picture is extremely well done; please don't think I'm trying to make mountains out of molehills. It's just that you've done such a good job that minor things are all you've left us to discuss!
Character design and posing: Excellent. The posing is very good, and as mentioned earlier, the reaction on Luna's face does an outstanding job of selling the moment. The overlap of Luna's elbows over Ce[Mo]lestia's forelegs is probably the strongest (and at once the most subtle) indicator that what happened was an unexpected conclusion to an ordinary (and welcome) hug; Luna is not pushing away or anything like that. It's obvious that the surprise is not the hug, but the kiss.
I do think Luna's right hoof-tip (the far one from the viewer) should be behind Celestia's necklace; Luna having both hooves to the right of Celestia's chest would cause a fairly obvious twist in her torso, but that twist isn't there in the rest of the drawing.
I'm a little surprised you didn't do more with Luna's wing-position; a bit of wing-splay would work well with the well-known trope, plus allow having a couple of Luna-feathers floating about as an additional "payoff" to the action.
Celestia's eye position does seem to me to be a little low-and-forward, and perhaps a little large. It may be show-accurate, but it nonetheless is a little distracting. Perhaps 10-15% smaller, leaving the top left corner of the eye-object where it is, would be a worthwhile quick experiment.
Celestia's mane, I think, could use a little more "sparkle". As it is, there are large areas of flat fill which are just a little on the plain side; in particular, between the two characters and behind Luna. Luna's mane is perfect.
Lighting: With the moon directly behind the characters, backlighting is going to be dramatic. You have the lighting locations correct, and what you have in place works just fine. That being said, I think pushing the contrast on the lighting would be helpful, as would increasing the shadow-intensity. Also, with a lighting-source as large in size as the Equestrian moon, the shadows' edges would be less sharp. A side observation: the moon would actually shine through the characters' manes, but I fully understand why that wouldn't work in this pic; too much "realistic" detail would break the overall cartoonish design sensibility. Having the manes be opaque is the right choice.
Background Very nice work. The level of detail is just right. The temptation to load the moon with craters and other detail must have been enormous. That being said, a little something more in the background might work... something far in the background flying in front of the moon. Discord? Philomena? A dragon? Nightmare Moon herself? Another possible embellishment is a fizzle of magic sparks from Luna's horn would add a little color, humor and impact to the picture. However, it's important to not get the moon too busy; I think you'd want to limit yourself to just one of the "background flier"/"loose feathers"/"horn-sparks". Managing the level of detail throughout the picture is a delicate balancing-act.
In summary, you've done an excellent job. This is a work to be proud of. Whatever people have to say about the subject matter, the art itself stands tall, and is a work of great skill. Well done.
I want to hug Luna right now, and I always want to hug/kiss Nightmare Moon. She did not expect that, and I doubt she's going to look her sister in the eye for a loong time. "I...Celly? I don't even like girls! I prefer the nerdy stallions, please let me g-Hmmpphh!"
I hate the fact that all these kind of things is only becoming more of every year and more for hating and mention it, and is disturbing that is all-things-smashed-into-one... Same sex. Family. Little-big. Animal etc. If there is more... But then again.. My lonesome brain/me likes what it likes, good work with, (you also make it seem not so bad as it is, and i can see the little humoristic part).