"What a great performance, Twilie!" Twilight's mother applauded. "You and your magic never cease to amaze me."
"If it weren't on your butt, I'd say that magic is your special talent!" exclaimed her father.
Twilight blushed mildly. "Thanks, Mom and Dad," she said. "How long are you two gonna be in Ponyville?"
"Oh, just for the night. We leave in the morning," her mother answered. "It's been a while since we've seen you, and we didn't wanna miss your performance."
"Are you going back to the motel now?"
"We are, Sweetie. It's been nice seeing you!"
The family shared a group hug before Twilight's parents started on their way to the motel. As they walked, Twilight got a glimpse of her father's cutie mark. Of course she had seen it time and time again throughout her childhood, before she moved to Ponyville, and of course she was aware that it was never changing and always a part of him. But never before had it ever stuck out at her like it did now.
"Hey-" she started to speak, but her parents were already out of earshot of her.
Twilight knew that she had seen a cutie mark extremely similar to her father's before. She felt as if she had just seen it. But where? Who bore her the mark on her father's flank?
Then, it hit her.
Trixie.
Twilight's pupils shrunk in her eyes.
"No way!" she said to herself aloud. She traced her memory back to when she first saw Trixie, and then to the fight she had with the Ursa Minor, while Trixie was stripped of her strange attire. Her cutie mark was clearly revealed. And it looked just like her father's.
"Hey, Trixie!" Twilight saw the unicorn hurrying off in the distance, tripping on her own hoofing as she stumbled to an escape. Twilight used a teleportation spell to catch up to her. "Trixie, wait up!"
Trixie stopped in her own tracks.
"What do you want, Twilight Sparkle? Haven't I done you enough favor considering all the humiliation I now feel as a disgrace to the ponies of Ponyville? So, what do you want? Another apology?"
Twilight shook her head. "No, listen- can I see your cutie mark for a sec?"
"My cutie mark?" Trixie asked. "Why do you want to see my cutie mark?"
"Are you apologetic or not?" snarled Twilight.
Trixie sighed, and, with her magic, removed her cape, revealing her pleasantly blue body and shiny, light-blue tail, cutie mark and all. Twilight studied it for a moment. It was nearly an exact replica of her father's.
"Finished yet?" Trixie asked, suspending her gown in the air with her magic.
"Yeah," Twilight answered.
Trixie redressed herself.
"Why did you need to see my cutie mark again?"
"I'll explain on our way to the Library. Mind staying in town a little longer, Trixie?"
"I...I guess not. I was just gonna flee to the Everfree Forest and live out the rest of my life hiding from the public in eternal humiliation for being an inferior unicorn, but I guess I can spare some time."
"Great. Follow me!"
On their way to the Library, Twilight explained to Trixie that she wanted to do some research on pony biology and genetic inheritence. The reason: Trixie and Twilight's father had nearly identical cutie marks. Twilight had a theory, but she needed literary proof.
"Hey, Twilight, hey-...Trixie?" Spike greeted the two ponies.
"Hey, Spike," Twilight greeted in response. Trixie made no remark.
"Twilight, what's she doing here?" Spike asked.
"I need Trixie for the moment; she's going to help me conduct some research on something very important."
"Y-you're not replacing me a-are you?" Spike stuttered.
"Of course not, Spike: you're my number one assistant! Speaking of which, could you fetch me that book on pony biology and genetic inheritence?"
"Absolutely!" Spike bowed, grabbed the ladder, slid it to his desired destination, climbed it, retrieved the book from the topmost shelf, climbed down, and handed the book off to Twilight, who used her magic to levitate it and bring it to her pedastal. "Now then." She started flipping through the pages. "Trixie."
"Hmm?" Trixie asked, approaching Twilight from behind.
"What do you remember of your childhood?"
Trixie pondered that inquiry. "I just remember growing up with my dad and my brother," she said. "Nothing specific. I didn't really spend a whole lot of time with them - or anypony actually. I was too focused on studying magic and becoming the greatest and most powerful unicorn in Equestria."
"I see...did you have a mother?"
"Well, of course, 'cause I was born, but I don't remember her face. I don't even remember meeting her. I never knew my mother."
"Fascinating...and what can you tell me about your father and your brother?"
"Um..." Trixie tried to remember, but she seemed to be extremely forgetful of her past. "I'm sorry, Twilight, but I simply can't tell you anything. My memory's all fogged up."
"That's okay. It's hard to remember things from your early to late childhood."
"Am I being of any use, Twilight?"
"Not really...but, uh...okay! I found something!" Twilight pointed to a paragraph in her book. "It says here that, as studies have shown, it is likely - but not certain - that offspring ponies will share the special talent of their parents. That makes sense, considering the Apple Family. All those ponies have cutie marks with pictures of apples and apple-related things on them. And their special talents all revolve around apple-related activities."
"What are you getting at?" Trixie asked.
"Haven't you noticed that the two of us share special talents in magic?"
"Well...now that you pointed it out to me, I can see that. We are both great with magic...you just...more than me."
"And, did you also know that your cutie mark and my dad's cutie mark are nearly identical?"
Twilight left the pedastal, and climbed the stairs to her nightstand and retrieved an old framed picture with her magic, bringing it downstairs as she descended the staircase.
"What? That's preposterous, Twilight! Your father and I aren't related in any way."
"Oh, yeah?" Twilight asked snidely, showing Trixie the portrait of her parents, both of her parents' cutie marks in clear revelation.
Trixie's pupils shrunk. She raised her cape over her flank and kept switching between looking at Mr. Sparkle's cutie mark and hers.
"That's quite a coincidence," she said. "The odds of your father and me sharing cutie marks and not being related are one in a billion, even with my special talent being magic."
"I don't think it's a coincidence," Twilight pondered.
"Really? Then what do you think it is? An anamoly?"
"You know what I think, Trixie?" Twilight asked, putting the portrait back where she found it. "I think you're my sister."