One of three things is going to happen:

A: Luna gets Celestia preggers
B: Greenhooves is going to tell the wrong pony what he saw, and whoever je ttells will put two and two together, or
C: both happen.

Just trying to cover bases here.

That ending, goddammit Greenhooves

There's a super-simple solution available if they do end up getting caught: lie. "Oh, we're not actually sisters by blood or anything, that's all just a convenient political fiction..."

6847142 You'll see

the law against it was strong

"Oh no Mr. Police Pony, please don't arrest the sun and moon goddesses, absolute diarchs who could change the law at any time."

Little did they know that in one moment, very soon… their whole fortune would tip.

"When the restaurant patron pulled the crayon out of his soup and set it down he turned his gaze once more to his delicious bowl of broth. He was shocked to find princesses Celestia and Luna humping right in his soup!"

- A Soupy Mess by NotARealFanfic

“Ok! Well, you kids have fun!”

"I'mma go be braindead over here now! Dumdeedaa!"

“Hmm, your royal bottom has gotten bigger by the way. Have you been eating cake ?” She teased as she turned to ME flirtatiously.

ME!? Hmmm...was this fic an entirely different story at one point?