Dear Princess Twilight,
How’s it going? We haven’t talked in awhile. I hope you haven’t had to deal with new threats to Equestria again.
Hi, Sunset! It has been a long time, hasn’t it? I’m sorry I couldn’t write to you as often this month. Everything’s just been so hectic! Especially since my brother and sister-in-law are taking care of a baby on top of everything else they do!
Oh, gosh. I don’t know a thing about parenting, but it can’t be easy.
Tell me about it. Flurry Heart’s Crystaller is especially worn out. He’s had to keep tabs on her magic 24/7. Can you imagine? Shining Armor decided to give him a break, thank goodness. He’s actually staying at my place for a while to recuperate.
Well, that sounds nice. You two know each other well?
Hmm, I’d say so. I mean, we’re good friends and all...but he’s mostly here for Starlight than anything else. She and Sunburst used to be best friends when they were young!
……..
Sunset?
Your niece’s Crystaller...his...name is Sunburst?
Yep. And also...don’t tell anypony, but...I think he might be a liiiittle bit smitten for Starlight, hee!
Excuse me?
Well, he might be, I’m not really sure. I don’t really have an eye for these things like Rarity does. But they do seem pretty close from my understanding.
My brother has a THING for this mare?
Wait, what? Your BROTHER!?
How dare she think she could ever be good enough for Sunburst!? You know what? I’m coming over! I’m coming over RIGHT NOW!
Sunset, WAIT!
In the library, Starlight yawned over her open book and gazed sleepily at Sunburst, whose nose was buried deep into his own book. “I don’t see how this is supposed to be…” She paused to yawn again. “...relaxing.”
Sunburst peered at her over his spectacles and chuckled. “Trust me, Starlight. Nothing can be more refreshing than studying the seventeen basics of teleportation!”
Starlight couldn’t help but roll her eyes. “I already know how to teleport, Sunburst! I don’t see how reading tons of chapters on the matter would help…” She suddenly lowered Sunburst’s book with a hoof, a smile curling her lips. “Why spend all your time reading about magic, when I could just show you?”
Sunburst looked almost crestfallen. “But I like reading!”
“I know,” Starlight replied, using her magic to organize the scattered books into a neat pile. “But you came to Ponyville to spend time with me, did you not?”
After a moment's thought, Sunburst relented with a sheepish smile. “That I did. I’m sorry, Starlight.” He stood up and stretched out his hind legs. “ What would you like to do today?”
Starlight glanced at one of the castle windows and beamed. “How about a picnic?”
“A picnic? As in...outside?”
“Aw, come on, Sunburst! With all those days you spend between the house and the Crystal Empire, I know that some fresh air will do you good! It helps clear your head! We used to sit outside every day in the afternoon...remember?”
The unsure look on Sunburst’s face slowly melted into one of fondness. “Well...it does look like nice day out there.”
“Atta boy,” Starlight simpered, patting him on the back, causing him to roll his eyes. “I’ll go get the picnic basket. Tell you what. I’ll let you take one book with you when we go out.”
“Ha ha,” he said dryly as Starlight trotted off. “I believe I can survive without a book for an hour!”
Suddenly, a flash of purple light startled him backwards. “What’s this about surviving without a book?”
Sunburst shook his head and blinked. “T-Twilight!”
“Why would Starlight make you go without boo--oh, what am I saying!? Sunburst, why didn’t you tell me Sunset Shimmer was your sister?”
Sunburst’s ears perked up and his jaw slackened. “S...Sunset? H-How did...you know her?”
Twilight paced back and forth. “It’s a long story. The trouble is, I kind of told her something she didn’t want to hear, and now she’s coming here!” Sunburst looked dumbfounded.
“Coming here? From where? She just disappeared years ago, I don’t see how--” Suddenly, a large object covered by a thick white sheet began to vibrate in the far corner of the library. “What in the name of…!”
Twilight’s eyes grew large. “Oh no!” she took flight and hovered above the object before pulling the white sheet off; a large mirror was revealed underneath. Bright magenta colors swirled and flashed in an unstable fashion. It buzzed and crackled more and more violently with each passing second.
Sunburst slowly walked toward it, his eyes glued to the device. “What...what is that?” he asked as Twilight landed beside him.
Twilight didn’t seem to hear him; she nervously rubbed her hooves together, mumbling under her breath and sweating. “Her first time back since the crown, and I’m completely and totally unprepared, so unprepared…”
All at once, a yellow blur shot out of the mirror and slid across the marble floor. Before either pony could react, the yellow blur knocked into them, sending everypony crashing into a bookshelf in a crumpled heap. A few books fell from the shelves and toppled to the floor. Multiple groans filled the air as the three of them tried to untangle themselves. “Someone’s glasses?” a voice called out, holding up a small pair of spectacles.
“Those are mine,” Sunburst grunted from under Twilight’s body, reaching up. He retrieved his spectacles and pushed them up his nose before locking eyes with a female unicorn. Her fiery red and yellow mane glinted off the bright lights in the ceiling, making it seem like it was glowing.
Her cerulean eyes widened with shock as she gazed down at the pony beneath Twilight. “Sunburst!” Sunset exclaimed. “Is...is that you? …Is that...hair on your chin?”
Before Sunburst could open his mouth, Twilight groaned and struggled to her hooves, allowing air to flow back into Sunburst’s lungs. The Princess of Friendship offered up a weak grin as she wobbled slightly. “Welcome...back!” Sunset steadied Twilight with a hoof before the alicorn fell over.
She chuckled at Twilight’s frazzled state. “Thanks…” She paused, taking in her friend’s physique. “So...this is you.”
Twilight looked down at herself and gave a laugh. “Heh...pretty much! And...this is you! To be honest, I didn’t really get a good look at you when you were...uh…” she trailed off, blushing.
Sunset smirked and patted her shoulder. “It’s okay, Twilight. I was too busy stealing the crown to get a good look at you either. But first…” A spark of anger ignited in Sunset’s eyes when she turned to her brother. “What’s this I hear about you dating someone?”
“Somepony,” Twilight whispered briefly.
“Somepony!”
Sunburst’s mouth opened and closed. “You were gone...for five years...and you’re concerned about who I’m dating!?”
“So you are dating somepony!”
“What--NO! I didn’t say--no. I’m not dating anypony! Whatever gave you such an idea?”
“Sunburst~!” Starlight sang, trotting into the room and levitating a woven basket. “I’m ready for our picnic together!” She halted when she saw three pairs of eyes trained on her. “Er...Hi?” She slowly put down the basket and looked to Sunset. “Who’s this?”
“Wait a minute…” Sunset squinted at Starlight. “You’re the kid with the pigtails that peed on the park slide!”
Starlight gasped and swatted Sunburst on the arm. “You said you wouldn’t tell anypony!” she growled.
Sunburst flinched and rubbed the back of his head. “I-It was just a little trickle! The slide still worked fine!”
Twilight looked between the two of them, her face scrunched up in confusion. “You...slid on pee?”
Starlight made a sound of exasperation, throwing up a hoof. “We were KIDS! PEE WAS PEE, NOPONY CARED!”
“I can’t believe you’re dating already,” Sunset said, looking slightly saddened.
Starlight’s pupils shrunk into pinpricks. “What?” Her head snapped toward Sunburst, who was blushing heavily. “We’re dating?”
“N-No! Absolutely not! My sister just thinks--”
“Sister? Oh, ponyfeathers. Now I remember you!” Starlight pointed an accusatory hoof at Sunset. “You turned me into an eggplant!”
Sunset lifted an eyebrow. “You wouldn’t get out of my room.”
“I’m so sorry I started this,” Twilight said feebly. She turned to Sunset and looked her square in the eye. “Sunset, nothing is going on between Sunburst and Starlight. I was just--”
“Well…” Starlight chewed her lip, looking up at Sunburst. “There was that…” Sunburst blushed even more, understanding. Sunset’s eyes widened, looking utterly horrified.
“What...did you do...with my brother!?” Both ponies quickly shook their heads, but Spike ran into the room before they could stutter out a response.
“What’s going on!?” Spike gasped, skidding to a halt between Starlight and Sunburst. “I heard ponies screaming about pee and…” He stopped and did a double take. “Sunset Shimmer? What are you doing here?”
Sunset’s hoof flew to her mouth. “Spike, you’re a...you’re a Dragon!”
Starlight snorted. “Did you hear that, Spike? You’re a Dragon.”
Spike laughed while Sunset shot Starlight a death glare. “I am? Well, nopony told me!”
Sunset stepped closer to Sunburst. “Just how long have you two been together?”
“Sunset, we’ve never been together,” Starlight said firmly. “Believe me, I had the same suspicions about Sunburst and Twilight!”
Sunset whirled around on Twilight as the purple alicorn started to back away. “Twilight!” Sunset cried, backing the poor Princess into a bookshelf. “Is that true? Why didn’t you tell me you were interested?” She paused. “I mean...I’m not judging or anything, but...Sunburst?”
“Hey!”
Twilight shook her head wildly. “No! For the upteeenth time, no! I’m not interested in Sunburst! H-Honestly, he’s more like a...brother! Heh…”
Spike frowned and walked over to Twilight, wrapping an arm around her fore leg. “If anypony is like a brother to Twilight, it’s me!”
Sunset quirked an eyebrow. “I always thought you were teacher-assistant.”
“I thought it was mother-son,” Sunburst said confusedly.
“I’ll never understand the relationship between you two,” Starlight mused. Spike promptly stuck his tongue out.
“Even if I was dating Twilight or Starlight,” Sunburst scowled, drawing everypony’s attention. “You would have no say in it! You lost that privilege when you skipped out on us!” Sunset’s ears flattened against her head, and an awkward silence filled the room.
“How about!” Twilight trilled, stepping out from behind Sunset and pushing Spike forward. “We go play some board games and give these two some privacy!”
“Aw, but I just got here,” Spike complained.
“Shoosh!” Twilight hissed as she ushered Spike and an uneasy Starlight out into the hall. She grinned unnaturally at the two siblings before closing the door behind her. The young Princess then slumped against the door and wiped the sweat from under her bangs. “Whew.”
But it wasn’t long before they heard muffled arguments coming from the other side. Curiosity taking over, both Spike and Starlight bumped Twilight out of the way and pressed their ears against the door, ignoring her indignant protests.
“Starlight GLIMMER? SERIOUSLY?”
“It’s just a coincidence!”
“Stop it, you two!” Twilight whispered sharply, using her aura to drag her friends away from the door. “This is personal business!”
“But that unicorn made it personal,” Starlight whispered back, frowning. “How dare she just come in here and accuse me of dating Sunburst?”
A small whimper emanated from Twilight, and Starlight's eyebrows furrowed. “Twilight...” she said slowly. “What...did you do.”
Twilight fidgeted before gulping loudly. “Well...y’see...heh..I kind of...sort of…” She cringed and shrank back. “I told Sunset that I thought he liked you...” Twilight flashed her most dazzling grin as Starlight’s eye twitched.
“T...TWILIGHT! Why would you do that! You broke the Girl Code!”
“...There’s a...Girl Code?” Twilight looked back and forth between Starlight and Spike, looking completely baffled. “I-Is it on record? Where can I find it?” She gasped, her hoof slapping against the side of her head. “Just how many codes did I break!?”
Starlight brought a hoof over her eyes and sighed. “Nevermind, just...nevermind.” Under her breath, she added, “I’ve been hanging out with Rarity waaay to much.”
“I’m an adult,” Sunburst said, irritated. “I can make my own decisions and I can certainly see whomever I want, with or without your permission!”
“Please don’t be angry,” Sunset said anxiously. “I know I left you, and I know I haven’t been the best sister, and I’m sorry. That’s not who I am anymore.”
Sunburst was at a loss. “Where did you go? What’s behind that mirror?”
The question caught Sunset off guard for a moment. As Sunburst stood there waiting for an answer, Sunset frantically wracked her brain for a suitable explanation for all this. How could she explain all the terrible things she did? The people she hurt? The worlds she almost destroyed?
Not yet, a little voice told her. Not yet...
Finally, she answered, “It’s a long story. Besides...you wouldn’t believe me if I told you.”
Sunburst fell silent for a moment. He slowly sat down on his haunches, and Sunset awkwardly did the same. The two sat for awhile before he broke the silence. “Why didn’t you come back?” Sunset didn’t miss the thread of hurt in his voice.
“At first...I kind of pushed all of you at the back of my mind. I was angry at the time...too driven to give my family a second thought. By the time I came to my senses...I don’t know. I guess I was too ashamed to even think about returning. I couldn’t bear the thought of facing Princess Celestia...or mom and dad. Not after what I had done.
"But I did think about you. I think about you all the time. I wanted to see you again. But... I figured that just because I wanted to see you, it didn’t mean that you wanted to see me. I kept thinking that you had moved on with your life, and you didn’t need me messing it up for you.” She gestured to Sunburst’s attire.
“I mean look at you...you’re tall now...and you have glasses...and a beard!” She giggled when Sunburst reached up to stroke his facial hair.
“I-It’s a goatee, actually,” he said, cracking a small smile. “And..you’re more beautiful than I remembered you.”
A tearful grin spread across Sunset’s face. “Sunburst...c’mere…” Sunburst sprang forward, and the two siblings embraced one another. Sunset buried her face into her brother’s cloak and squeezed tightly. “I’m sorry,” she whispered. “You just grew up so fast…”
“So did you,” Sunburst whispered back. He closed his eyes and rubbed her back gently. “I missed you.”
Pulling back, Sunset planted a small kiss on Sunburst’s forehead. “I missed you too. We have a lot of catching up to do. But first…” She smirked. “Why don’t you tell me how you really feel about Starlight?”
“I...well..that is to say...um….”
The library door opened, and Twilight, Starlight and Spike stepped back in alarm.
“We weren't listening!” Spike blurted.
Sunset strolled past Starlight. “Let’s talk,” she said lightly. “You and me.”
Starlight suddenly felt quite ill. “...Oh boy…”
“So,” Sunset said casually as she and Starlight walked down the long halls of the Castle. The older unicorn lifted her eyebrow and fixed the mare beside her with a cool stare. “Do you think you’re Sun-worthy?”
Eyes darting about, Starlight chose her words carefully. “I...think so? I mean, we’ve already made up a long time ago, and we’re pretty good friends now…”
“In her letters, Twilight mentioned that you’ve had...an interesting past. Would you...care to elaborate?”
Starlight sighed and stared off into the distance. “It all started when I was a small child…”
Two Hours Later...
By the time Starlight finished, they were well into the Castle’s Friendship Dungeon. Sunset Shimmer exhaled. “Wow. That’s...that’s something.”
Starlight smiled sheepishly. “Ahah...yeah.” The two of them turned around and started back up the stairs.
“I mean, that’s way farther than I would've gotten. I mean, you even escaped at first! Old me would've been impressed.” Her expression turned grim. “But I’m a different person...pony now. And Sunburst is...everything to me. If anything, your past is alarming to me.”
Starlight lowered her head. “I know.”
“But...” A firm but gentle hoof rested on her shoulder. Starlight slowly relaxed upon Sunset’s empathetic gaze. “It would be hypocritical of me if I didn’t give you a chance.”
She lowered her hoof and smiled warmly. “I’m really proud of him, and you. Last time I saw you two, you were just a couple of kids in a sandbox. Now he’s a great Wizard and Crystaller to the new Princess...and you’re a prized pupil of Twilight’s!”
Starlight blushed. “Well, I don’t know if prized is the right word...but I’m getting there. At least I would hope so.”
Sunset nodded. “If anypony were to take good care of Sunburst...it would probably be you.”
“Probably?”
Smirking, Sunset replied, “I didn’t say I’d trust you all the way. He is my little brother, you know.”
“I’m anything but little!” a voice huffed.
After sharing a glance with Sunset, Starlight released a burst of magic from her horn. As magic showered around them, Sunburst, Twilight and Spike appeared beside them with sheepish grins. Starlight walked up to them with an air of mock superiority. “Eavesdropping, are we?”
Twilight offered up a quivering grin. “I was weak!”
Sunset’s horn glowed, and Spike was suddenly lifted into the air. The little Dragon was alarmed at first, but soon gave a joyful whoop as he sailed across the air in a superhero fashion. Sunset’s face lit up as she flew Spike above their heads in a circle. “It’s been a long time since I used magic like this!”
Meanwhile, Sunburst took a tentative step toward Starlight. “So...uh…about that...last visit…” He trailed off, too embarrassed to finish his sentence.
Starlight suppressed a giggle. I’ve gotta admit...he’s kind of cute when he’s blushing. “We never really talked about that, did we?”
Sunburst’s eyes shifted away from her for a few moments before redirecting his gaze. “Do you...want to?”
Starlight tilted her head, a smug look crossing her features. “How about we talk...over our picnic?”
Sunburst seemed to brighten up at that. “I’d like that...I’d like that very much.” He turned his head. “That is...if my sister doesn’t mind.”
Setting Spike back down to the ground, Sunset looked between the two of them and nodded. “It’s really up to you, Sunburst...but you have my blessing to do whatever it is you want to do.” Her eyes suddenly widened. “Well, not everything. Forget I said that.”
“In the meantime, you’re more than welcome to stay here for a while,” Twilight suggested, coming up beside her. Sunset’s eyebrows lifted.
“Are...are you sure? I don’t want to impose or anything.”
Twilight giggled, draping a hoof over her shoulder. “Don’t be silly. We’d love to have you here.”
Spike climbed on Sunset’s back and grinned. “Alright! Round up the others, and we could have a giant sleepover!”
“We’ll see, Spike.”
Starlight grabbed Sunburst by the cloak and started to pull him in the direction of the main entrance. “Well, come on, then! I know the perfect spot…”
Sliding across the floor, Sunburst managed to turn his head and wave. Sunset waved back, her eyes filled with love and pride. “So..” Twilight murmured softly as the pair trotted off. “Does he know about…”
“No.” Sunset sighed. “I think It’s better that way.”
“He’ll understand,” Twilight said. “He loves you, no matter what. If we could forgive you, Sunburst certainly will.”
“But I haven’t forgiven myself,” Sunset said earnestly. “Not completely. Until I do...it’s best that I tell him when the time is right.”
Twilight turned to look at Spike, who still sat on Sunset’s back. He gave a small nod, conveying that he understood. “Well,” Twilight said in a lighter tone. “It’s your first day here. Why don’t I show you around Ponyville?”
The corners of Sunset’s mouth lifted. “That sounds great!”
As they began to walk, Spike spoke up. “Sunset? Do you...do you think you could maybe come with us to visit Princess Celestia some time?”
“....Maybe, Spike. Maybe.”
“Starlight!” Trixie exclaimed as she and Sunburst emerged from the Castle. She trotted up the steps and hugged her newest friend. “I was just about to come and see if you were available! Trixie is thinking about doing an amazing underwater escape act that you’d be perfect for!”
Starlight’s ears perked up in interest. “That sounds dangerous...I like it. But, maybe next time. Sunburst and I are going on a picnic.”
She gestured to the stallion beside her, who gave a small wave. “Hello there!”
“Hello yourself,” Trixie replied.
“You go on ahead, Sunburst,” Starlight said before turning back to Trixie. “Maybe we can reschedule?”
“Oh, of course. Trixie can wait…” Her eyelids lowered as she watched Sunburst trot down the steps. “Who’s your friend there, Starlight?”
Well aware of that much-too-invested look on Trixie’s face, Starlight gave her a warning glare. “Trixieeeee….”
“Aww, you’re no fun.”
Author's Note
So, here's my head cannon.
After Sunset left for Canterlot, Sunburst was inspired to be just like his big sister and worked hard to know everything there was to know about magic. But by the time he got accepted into the same school, she had already left for the human world. What do you think? 
So yeah! Spike, ShimShine, GlimGlam, Sparkle and Burst. One big happy family. 
Critique is greatly appreciated 
