Rarity, when she really got into her work, often found it hard to really notice anything else going on around her, and today was no exception. As she sat at her desk, in her private workshop, working on her latest creation, her focus was like a laser, never straying from its target. Materials and tools were strewn about her desk, and she didn't even need to glance at them to know that they were there. She just reached out and picked them up as soon as she felt she needed them. Her eyes never once lifted from the dress itself, which was forming as if out of thin air through the sheer ability she wielded. Yet, as she did this, she failed to note the squeak of the door as it opened, and the sound of small hooves coming in from behind her. No, her work was her whole world right now, and everything else was little more than distraction, like when one was reading an engrossing book and ignoring the sound of rain upon the window. There came a time, however, when her attention was broken by the sound of somepony clearing their throat, and only now did she finally realise that she was no longer the only one here. Casting a glance behind her, a smile formed on her lips when she realised who her visitor was.
"Sweetie! Oh, how delightful! Come to watch your sister create works of art, perhaps?"
The young filly smiled, but it was a nervous one.
"Um...I did want to see you, but...it was about something else."
Rarity, still smiling, looked back to her work, getting down to a particularly delicate bit of sewing.
"Well, feel free to say what you need to, dear sister. I'm always open to listen."
Sweetie approached, but hesitantly. Something was on her mind, that much was obvious, but it was also obvious that she was struggling to get it into words.
"Um...Rarity? You know a lot about...well...mares and stallions? I mean...what kinds of stuff they do together?"
Rarity, though briefly taken aback by such a question, soon resumed her smile, though it came back a touch more slyly than before.
"Well, far be it from me to brag, but I suppose I do have some knowledge on the matter."
She raised an eyebrow to her sister.
"Why? Something you and your friends have been talking about?"
Sweetie shook her head, then suddenly started to blush, turning away so she wasn't looking at Rarity anymore. Rarity, perhaps finally sensing that something was up, stopped what she was doing, and gave her sister her full attention. But she didn't say anything, instead waiting for the filly to speak for herself.
"It's just...er...there's...this...boy."
Immediately, understanding came to the fashionista, and her eyes widened with shock. Sweetie, now knowing that her sister understood why she was here, simply stood there as her blush grew. Rarity, after wrangling with this revelation, eventually put on the biggest smile her sister had ever seen on her, and lit up her horn. Sweetie soon found herself unceremoniously yanked off the ground by Rarity's magic, and brought to her to be embraced in a tight hug.
"Oh my goodness! My little sister has found somepony special! Oh, what a joyous day!"
Letting go of her slightly, Rarity began asking a barrage of questions.
"Who is it? Rumble? Snips? Snails? One of the other colts in your class?"
Sweetie, after struggling to get herself free, shook her head at all of hose suggestions.
"No, it's none of them! It's...well...it's someone who doesn't actually go to my school."
Rarity, now intrigued, tapped her chin.
"Ah! So it must be somepony who's just moved into town, yes?"
Sweetie nodded.
"Well...yeah, he has just arrived in Ponyville."
Eagerness gripped Rarity, and she merrily clapped her hooves together.
"Ohohohohohoho! Who could it be?"
She moved off, looking out of her window.
"Now now, don't tell me! It's...um...I think a new couple moved in with a colt about your age earlier this week and..."
"It's Spike."
Silence fell, and Rarity slowly turned, blinking in the same way one would when they hadn't quite come to terms with what had been said to them. Yet as she looked to her sister, she saw that there was no lie there. She had indeed said what she'd said, leading to some understandable surprise on Rarity's part.
"...Spike?"
Sweetie nodded.
"...As in...the Spike who lives with Twilight?" Rarity enquired further.
Again, Sweetie nodded, and this time, Rarity took a step closer to her, sounding like she was still uncertain about this.
"Well...this is certainly a surprise."
Calming down from both her present shock and her earlier rampant curiosity, Rarity sat beside her sister, holding her close as she spoke again.
"And how did this happen?"
Sweetie sighed, looking to one side as she began to explain.
"Well...it all began when Twilight decided to set up a play date with me, Spike and the other girls."
Rarity nodded.
"Oh yes, I remember. She wanted him to start making some friends his own age. And how did it go?"
Sweetie smiled, but only a little bit.
"It was a lot of fun! We played hide and seek, raced against one another. It was great!"
Here, she faltered, looking away again, which naturally led to Rarity leaning closer to her.
"Buuuuuuuuuuuut?"
Sweetie faltered, but started to speak anyway.
"Well...we were chasing each other again after a bit. Bloom was chasing Scoots and Spike was chasing me. It was all fun, but...then Spike tripped on a root in the ground."
Her blush returned.
"I stopped to see what was going on, and when I did, he...he fell on top of me. We both fell to the ground together and..."
She covered her mouth, as if some terrible secret were about to be revealed. Rarity was waiting, anticipating what was coming next.
"And?"
Sweetie, mustering her courage, looked her sister right in the eye and answered.
"And he kissed me!"
Rarity paused, as did Sweetie, and the two sisters sat in silence for some time. After a few minutes though. Rarity looked to her with an arched eyebrow.
"He...kissed you?"
Sweetie nodded, and Rarity spoke again,.
"When he fell on you?"
Again Sweetie nodded, and again Rarity questioned.
"So...you two kissed...because of an accident?"
Sweetie pulled away from her here, looking down at the ground in shame.
"We didn't mean for it to happen! But...it did happen! And now...now we're special someponies to each other!"
Standing up, Rarity looked to her sister, half-confused and half-amused.
"Oh, you are, are you?"
Sweetie looked to her, almost astonished she'd even ask
"Of course we are! That's how it works! When a boy and a girl kiss it means they're a couple!"
She looked away again.
"I never even thought about having a coltfriend before, and I never thought that Spike would be that coltfriend but..."
She released a long sigh.
"...I guess that's what he is now."
Another silence, but eventually, Sweetie's ears perked up when she began to hear, of all things, snickering. Turning, she saw that it was Rarity, who was doing her best to hold in a laugh of some sort. Of course, Sweetie Belle frowned to this.
"Rarity! This isn't funny! I've just got into a relationship and I didn't mean to! You shouldn't laugh!"
But Rarity, wiping away a laugh-induced tear from her eye, walked to her sister, reaching out and holding her again.
"Oh Sweetie! My dear, beloved Sweetie! You're not in a relationship!"
Sweetie, blinking with confusion over that, looked to her sister like she was insane.
"What are you talking about?! We kissed! That means we're coltfriend and fillyfriend now!"
Rarity stifled another laugh, nuzzling her sister affectionately.
"Oh dear. I realise I should've explained these sorts of things to you a lot sooner, but simply kissing doesn't mean you're in a relationship now, especially if it was by accident."
Sweetie looked to her a lot calmer now.
"It...it doesn't?"
Rarity shook her head.
"No, dear thing. Relationships don't just happen with a kiss. They happen only when the two want it to happen. You and Spike are friends, Sweetie. Do you want him to be your coltfriend?"
Sweetie considered that.
"Well...I like him. He's fun to be with."
Rarity raised an eyebrow,.
"I didn't ask if he was fun, Sweetie."
Sweetie paused again, and her face softened.
"...No. I don't want him to be my coltfriend."
Rarity nodded.
"And does Spike want you to be his fillyfriend?"
Here, Sweetie stumbled, which was understandable, given that it was often harder to understand the mind of another person. But, in the end, she still answered.
"...No. I...I don't think he does."
Rarity smiled, kissing her on the forehead.
"In that case, sweet sister...there is no relationship."
Sweetie sat there, stunned by this new knowledge, and Rarity couldn't help but smile to the youthful misunderstanding and naiveté that she had witnessed. But she held her sister close.
"Sweetie, I'm happy you came to me to talk about this. I appreciate you see me as somepony you can talk to. But please understand, a simple accident, or touching of lips, does not a romance make. If what you and Spike want is to remain as friends, then that is what will happen."
Slowly, Sweetie started to smile, looking to her sister happily.
"So...we don't need to worry about getting married just yet?"
Rarity rolled her eyes, but kept her smile as she again regarded her sister.
"No, Sweetie, you don't have to worry about that."
Sweetie was overjoyed that this imaginary burden she'd placed upon herself was now lifted, and she eagerly embraced her sister. It was a nice and tender moment, but one that was soon interrupted when, all of a sudden, there was a knocking at the door. Turning, both sisters looked on with surprise, as neither was expecting any visitors. Yet, they both went downstairs together, heading for the front door, which Rarity soon opened. There, to their further surprise, was Twilight, and beside her was a very sheepish-looking Spike. The young dragon blushed when he saw Sweetie, and she, in turn, blushed to him, but as endearing as that was, Rarity's attention was on Twilight, who now spoke.
"Rarity...I'd like some help talking to Spike, if that's okay."
Though Rarity had a good idea what this was about, she nevertheless smiled down to the young dragon.
"Oh, and what might that be about?"
Poking the ends of his fingers together, Spike slowly answered.
"Um...well...do Sweetie and I need to get married now?"
Rarity sighed, shaking her head slightly while keeping her smile.
"Oh dear...you children really are too adorable for your own good sometimes."
Author's Note
It's something that occurred between some of my classmates during my very early school days, and something that, from what I've heard here and there, has actually been an experience several others have had in their youth. So, figured it'd make a sweet little story 
