(I’m gonna guess that this won’t be the only story with this title :P )
“Oooh, I can’t wait!” Pinkie Pie exclaimed, bouncing up and down in place. “This is gonna be like the coolest, most awesomest thing EVER!” The others all nodded. They might not have been matching Pinkie’s exuberance, but the fact remained that they were quite excited for the day’s events.
“Sure is excitin’, Pinkie,” Applejack concurred, looking around. “But don’tcha reckon Rainbow Dash shoulda been here by now? S’almost 1 PM, an’ this here gatherin’s mostly fer her anyhow.”
“I’m sure she’ll be here very soon,” Twilight said. “I can’t see why she’d possibly want to miss this!”
“Rainbow Dash does tend to be late for her appointments,” Rarity said, looking at her hoof. “You would think that Equestria’s fastest pegasus would be able to arrive for plans on time, especially plans like this.”
“Um…I’m sure she’ll be along any moment,” Fluttershy said quietly. Suddenly, all eyes turned to Fluttershy.
“Fluttershy!” Pinkie said, gasping. “You must know something about this!”
“What do you mean?” Fluttershy asked, tilting her head.
“Pinkie does have a point,” Twilight said, looking ponderous. “You’ve known Rainbow the longest, right?”
“Ever since we were little fillies in flight school,” Fluttershy responded.
“Well, certainly you must know what her twin brother is like!” Rarity said, realizing what Twilight was getting at. Fluttershy looked away and shook her head.
“…I didn’t even know she had a twin brother,” Fluttershy said. “I’m sorry…” The others looked at each other, confused. Before any of them could say another word, Sugarcube Corner’s front door opened, and the mare of the hour flew in, looking quite bothered.
“DASHIE’S HERE!” Pinkie shouted, pouncing her as she entered.
“Whoa!” Dash said as she was bowled over. “Easy, girl.” Pinkie gigglesnorted and got up, letting Rainbow pull herself to her hooves. “I’m not even late or anything, sheesh.”
“Rainbow Dash,” Twilight said, trotting up to her. “We’ve all been a bit curious about something.”
“Yeah?” Dash said nonchalantly. “What?”
“Well,” AJ spoke up, stepping up to join the others. “We were all mighty baffled ‘bout this whole twin brother thing.”
“What’s to be confused about?” Dash said, looking away. “I have a twin brother. He’s coming to visit. How can that be confusing?”
“Well…” Twilight said, “We were a bit confused about why you’d never mentioned your twin brother before.”
“You mean like how you never mentioned Shining Armor before we got the wedding invitation?” Dash asked curtly. Twilight opened her mouth for a retort, then fell silent. She had her there.
“Well, darling,” Rarity said, joining the others as well. “It still does seem a bit odd that he has never come up in conversation.”
“I just don’t talk about everything, okay?” Dash said, sitting down in a nearby chair. The other ponies followed her, like moths to a flame.
“Why wouldn’t ya talk ‘bout a twin brother?” Applejack asked.
“Yeah!” Pinkie chimed in. “Having a twin’s super-duper awesome! Or at least I’d think it is! I don’t have a twin! But I do have triplets, and that’s REALLY awesome, so having a twin must be at least half as awesome! Or would it be a third as awesome? Ooh, or maybe it’s 100% more awesome since-“
“Pinkie,” Twilight said, turning to her. Pinkie smiled and zipped her lips; she knew that “That’s enough” look from Twilight by this point.
“I just don’t talk about it!” Dash said, beginning to get a bit annoyed. “Do I have to talk about EVERYTHING with you girls? I’m sure there are things you don’t talk to us about. Right, Applejack with her subscription to Playpony Magazine?” AJ turned a bright red and tried to hide her face behind her hat.
“There’s no reason to toss secrets around, Rainbow Dash,” Twilight said in a calming tone. “We were just wondering. I mean, even Fluttershy doesn’t know about your twin brother, and she’s known you since you were young!”
“He moved away really early, okay?” Dash said, looking at the ground. “He couldn’t live in the kind of environment our home had. That’s probably what your little psychoanalysis books would say, right Twilight?” Twilight sighed. It DID sound like something her psychoanalysis books would say, but she didn’t want to admit it.
“What was wrong with your environment?” Rarity asked.
“Nothing!” Dash responded, gritting her teeth. “There was nothing wrong with my home! He went off and lived somewhere else with a nanny, we’d go to visit him and stuff…look, you’re gonna meet him in a minute or so anyway, so let’s just drop the issue, okay??” The girls looked at each other. Twilight began to speak, but was interrupted by the front door opening.
“I hear my sister Rainbow Dash is in here?” the pony in the doorway asked. Dash stood in the back of the room, not saying a word. Pinkie was the first of the ponies to respond.
“RAINBOW DASH’S TWIN BROTHER!!” Pinkie shouted, as she pounced across the room. She somehow skid to a halt in midair just before colliding with him, and held her hoof out for a shake.
“I’m Pinkie Pie, one of Rainbow Dash’s bestest friends!”
“I’m Rainbow Blitz,” Rainbow Blitz responded, accepting the hoofshake. “Nice to meet you!”
“Nice to meet you too!” Pinkie exclaimed, beaming. “So you must be a pretty good flier too, huh?”
“Uh…Pinkie…” Twilight tried to interrupt.
“Because Dashie’s all like ZOOM WHOOSH FLYYYYYYFLYFLYFLY and stuff!” Pinkie continued.
“Pinkie…” AJ spoke up, but Pinkie wasn’t listening.
“Did you two do synchronized flying stunts?!” Pinkie asked. “Ooh, that must be so awesome…”
“PINKIE!!!” Rainbow Dash shouted at the top of her lungs, a shout that finally garnered her attention.
“What is it, Dashie?” Pinkie asked, turning to look at her friends. All five ponies pointed back towards Rainbow Blitz. Pinkie turned around and took a closer look at the pony standing before her, who was now looking quite uncomfortable. She stared for a good long while, until her eyes lit up.
“OH!!” Pinkie said with a giggle. “You’re an EARTH PONY!!” Blitz smiled awkwardly. Dash said nothing. She just turned away.
Norse Pony’s comments: Ooh, a nice take on the prompt. Dash’s reluctance to address the fact that she has an earth pony sibling speaks volumes about her and about pegasus mores. It’s making worldbuildy things happen in my head. I liked this a lot.