Don't DO that, Lulu!
Celestia had many happy memories with her sister; the most memorable of those were the times where they would get into food fights with one another over extremely petty reasons (Such as which sister had more chocolate, and Celestia won most of these food fights), quite a few memories of reading some of the very first books to Luna when she was the equivalent of a small filly, and even a memory or two of tucking Luna into bed. Now those were the good memories, and she was often reminiscing the good old days whenever she had some time alone.
However, with those good memories also came bad ones; ones that Celestia did not like to reflect on, yet did anyway, almost as often as she would her good memories. These memories mostly consisted of Discord, though they didn't faze her much, as Discord didn't actually harm anypony directly; no, the memories that did bother her the most were the ones of her sister before her fall to Nightmare Moon; everyday, at some point in the day, she would find herself looking up at the general direction of the moon, and wondering if she could have had stopped her sister in another way that didn't end up in her banishment, and with her being alone for a thousand years.
As she was in the middle of thinking of Nightmare Moon once again, she was unaware of Luna herself stealthily sneaking up behind her, with a brown paper bag in her hooves, already blown up and full of air, ready to pull a fast one on her. Luna had seen this action being done to guards in the mess hall at times, and she wondered how her sister would react to it; she didn't seem like the kind of pony to be particularly jumpy, but you never knew what would happen to a pony until you tried something on them!
Luna continued to creep up on her unsuspecting sister, taking note that she seemed to be in the middle of a daydream; perfect, just perfect. Her slow hoofsteps made no noise as the lunar princess continued to slowly close the distance between them, with the evil intent dead set on her mind; her sister would most likely call for unrelenting retribution on her, but it would be worth it in the end, with her getting the last laugh as her sister would do something unspeakable, like smacking her across the face with a pillow. Full of bricks. While on fire.
She raised up the bag and a hoof, preparing to smash her hoof into the bag, ready to startle an oblivious Celestia...
When a voice sounded out cheerfully at the worst absolute time. "Auntie Celestia, auntie Luna, hi!"
Many things happened simultaneously, and with hilarious results that were not expected by anypony present at the time, and, once the initial rage and awkwardness from this event faded away, it became a rather funny and only slightly embarrassing memory for those involved.
The voice had come from behind the two alicorns, and naturally, Celestia had turned around to address the speaker, only to be confused by the sight of Luna holding a brown bag to her face. Naturally, her response was to wonder what the heck was going on.
"What th-"
At the same time, Luna panicked, and having nowhere to hide the blown up, brown paper bag anywhere quickly, decided to detonate it with her hoof and taking what she could still salvage from her ruined plan, causing what would later then be recorded as the loudest explosion to have come from a brown paper bag in history. This explosion also happened to be in her sister's face, who, after receiving the blast, began to flail her hooves wildly in a panic, believing to have been shot; this caused Luna's nose to get hit with a hoof, and the lunar princess fell back on her rump with a yelp, barely managing to not get trampled by her sister. Celestia then somehow managed to fall on top of her sister, with her hooves on her ears, trying to get rid of the ringing noise that was in her head.
This was all observed by a now speechless Cadence and her husband, Shining Armor, both ponies looking down at the normally dignified princesses in a moaning, pain filled princess heap. Shining Armor looked confused, and was about to say something when Cadence touched his shoulder; he looked up at her, and saw an equally confused Cadence looking at the alicorn pile in front of them.
After a short moment of watching her two aunties in a dazed stupor, Cadence realized that the two weren't going to be getting out of their confusion and pain anytime soon, so she decided to give them a helping hoof. Cadence walked up to Celestia, and gently tapped the base of Celestia's horn; she repeated the action with Luna, and stood back as the two shook their heads, the confusion gone from their minds from the simple action (Something that Cadence learned: Tapping the base of a unicorn's horn was very effective in dispelling any headaches). Celestia, naturally, was first to speak.
"Don't DO that, Lulu!" Her sister was quick to respond, with a sheepish look towards Cadence and Shining Armor that was just screaming 'help me'.
"Ah ha, ha ha...What are you talking about, Tia? I didn't do anything! It must have been a...Ah, trick of the light! ...Get it?" Unfortunately for Luna, Celestia was not about to let her off easily this time, and growled into her face, causing Luna to give a quiet 'meep' of distress.
"Don't play games with me! You exploded a bag in my face! What were you thinking?!" A cowering Luna took a moment to mentally think up of an explanation, and offered a slight smile at her sister as she thought of a very bland explanation, which wouldn't win any awards, but would (Hopefully) convince Celestia that she was not at fault.
"W-Well, I..." Her words were cut off by a suddenly calm sounding Celestia, who sighed and nuzzled her sister gently.
"You know what, forget it, Lulu." Luna blinked, and tilted her head slightly in a questioning manner; this wasn't something she expected her sister to say!
"...Does this mean I'm not in trouble, Tia?"
"Only if I can't catch you first." It took a short amount of time for Luna to realize what her sister meant by that, and her eyes widened with dismay and horror.
"...Oh, Faust, help me now!"
Luna pushed Celestia off of her, then scrambled to her hooves and darted away quickly, followed by Celestia, who was toting not one flaming pillow filled with bricks, but two; for the next five days, Celestia would be running after Luna nonstop with an impressive array of bedroom themed weaponry. As Luna was running for her life, Cadence sighed, and looked at her husband, who looked back at her.
"Well...Okay, then. Shining?"
"...Yeah, Caddy?"
"...Want to get a milkshake? I'm sure those two will have fun with each other for a while before realizing that I'm here with you. Might as well get a milkshake or two before they say hi to me!"
"...Aren't you afraid they're going to kill each other? I mean, they're alicorns that can sling magic five miles away to create a crater the size of Manehattan. With a sneeze." Cadence shrugged, and looked over towards the general direction of where the castle kitchens were, slightly drooling over the prospect of a milkshake.
"Eh, I'm not that particularly worried about either of them, to be honest."
"...Are you sure?"
"I think Celestia is content with whacking Luna over the head with a flaming pillow filled with bricks rather than blast Luna with some super powerful end of the world two by four laser beam from her mouth mystical mumbo jumbo magic flare that would probably kill us all. I'm sure Celestia wouldn't kill everypony, much less Luna!"
"...If you say so; In that case, let's go get some milkshakes, Caddy!"
The two then began to walk down to the castle kitchens, narrowly managing to avoid a flaming brick that was meant for Luna as the two older alicorns darted around, one fleeing for her life, the other intent on catching her sister and doing unspeakable things to her with lipstick.