Headcanon backstory: The reason why Luna and Celestia still like to have tickle fights in their adult years is because in their younger childhood and teen years they would always have playful tickle fights and they never outgrew it.

Vice President Luna: Come on, Tia! Giddyup! Giddyup! Giddyup!  
Principal Celestia:(happily) Neigh!  
(Principal Celestia walking hopping like a pony, carrying Vice Principal Luna on her back, holding Principal Celestia’s hair like a pony’s mane hair like a reins)  
Vice Principal Luna: Rear up and neigh like a good horsey!  
Principal Celestia: (rearing up) NEIGGGGGGHHHHHHHHHHHH!!  
Vice Principal Luna: You’re a good horsey! (shaking Principal Celestia’s hair) Good horsey! You’re a good girl! Such a good horse! Let’s get you some big juicy red apple.  
Principal Celestia: (turn around and rearing up) NEIGGGGGGHHHHHHHHHHHH!  
(Principal Celestia starts running through the door open and heading to cafeteria, carrying Vice Principal Luna, holding Principal Celestia’s hair, and Vice Principal Luna laughing)

Luna wanted a salary increase but Celestia denied her sister that wish.
 
Now the principal has to learn the hard way why it is a very bad idea to say no to Luna.
 
She is punished by merciless tickling while her baby sister is gloating and teasing her.
 
Luna won´t stop until Tia accepts defeat, submits to her, begs her to stop and implores her to accept the salary increase.
 
As further punishment Tia also has to work as Lunas slave assistant and do everything she orders her for one month.

@Background Pony #0F9E
 
Even so, it’s not even incest in the picture so it’s not princest. Just because some people find this hot doesn’t make it incest.

As much as I like seeing Luna receive tickles, the notion of the little sibling eventually be big enough to dish out payback on the big sibling for years of teasing is always great.

@headlessrainbow  
People with fetish is the target audience.

Nice feet and shadowplays i hope luna is next :)

How is pinning down your sibling and tickling them considered lesbian incest?

Haha, “poor” ticklish silly Celly! You should reach to your sides Celly, knock Lulu’s pumps off loose and tickle her upturned soles! I wonder if she’d be able to maintain her dominance if she was being tickled just as much in return? ;D

Is it really princest is there’s no hint at a relationship whatsoever and nothing explicit or even suggestive?

Now lick.

oh my god this exists :)

Very very very cutie!!!