This is everything I have ever wanted in a fic. Thank you for being awesome.

Twilight as a concubine/pet is a nice premise. However, this is in deep need of an editor, especially punctuation as you need a whole lot more commas, but there are also more than a few spelling typos. Lots of run on sentences.
The relationship between Celestia and Twilight, the threats of punishment, are also an interesting touch.

The only other pony who could even claim to reach Twilight’s level of popularity with their princess was a particular earth pony stallion who claims to be the only pony alive to be able to take the full wrath of their princesses’ massive endowment in a hole that wasn't their mouth.

Claims? One would think Celestia would have brought in multiple concubines at once to tend to her entire body, allowing them to see first hand what the others can do, maybe even do it with each other while Celestia watched.

7780740 This is correct. There are capitalization errors that need fixing as well.

Also, you reference an earth pony stallion at one point, but imply later that Twilight is thinking of Flash Sentry. Flash is a pegasus.

7782161
Fixed, thanks!

Usually i am all for a love between equals, but the dynamic here is really intriguing.
Also, this sentence encapsulates all my feelings about a certain character:
"Beat that Flash Sentry you gay bottomless slut and your stupidly massive ass!"
May he stay away from Twilestia for eternity, the stupid git!

Please please please please PLEASE proofread. You have a lot of improper apostrophe use, many mixed up uses of your/you're, and some awkward sentence structure. Now that that complaint is out of the way, this is the hottest story I've read on fimfic in a long time and I am following you now. Well done.

This is 100% awesome beyond awesome read.

So immense was the pleasure radiating off the huge amount of cock resting inside her that she nearly came the instant her lips pressed against against the familiar white balls.

She stuffed the thought away quickly and returned to serving her mistress, putting the thought the of the only other concubine she had ever been jealous of behind her.

The genuine concern in the usually stoic princesses’ voice made twilight’s guilt flare up.

or I’m afraid I might be asked some rather awkward questions.