@Background Pony #78F6  
Well, I gone with “assuming the artist motive”, because of my bellow comment, and based on the tags, seeing I saw “lunestia”, used sactually more often than princest to name the Celestia x Luna pairing. Iagree though it isn’t much implied by the picture though, although Luna and Celestia, don’t seem disgusted, rather irritated, and towards a character, or situation not visible in the picture. Although I think now it may have sense to remove the shipping tags, if it isn’t portrayed in the picture or isn’t (at keast as of now), part of a larger shipping picture-storyline.

@BackgroundLoL  
There’s nothing in the picture inferring anything incestuous between them, so even if the artist used those tags on DA, that doesn’t mean they were using them correctly, and assuming their comment means they like them as a romantic ship is a stretch to say the least. They could simply be saying “I like how it turned out” or something to that effect. They’re not even making physical contact and look pretty disgusted. This doesn’t really look like an incest image unless you try to apply some assumed artist motive.

@neo4812  
Well, I explained bellow XD
 
And yes, Luna and Celestia are very cute here! >w<

why is this tagged as incest? there not doing anything. they are cute though

@Background Pony #78F6  
Well, on the source pic the tags by the author read “#celuna” and “#lunestia”, alternate names for princest, so the author does seem to see it as a shipping pic, especially that she wrote in the description “I love it (aparently princest/lunestia) too”.  
I added the “principalcest” tag, as those are obviously meant to be younger Principal Celestia, and Vice Principal Luna.

So, why is this tagged incest, princest, and principalest?