So! I am SUUUUUPER excited to write this oneshot for multiple reasons! But the main reason is because this is going to be a bit of a crossover...

In this oneshot, you are Olivia Aphrodite Holmes, Sherlock's little sister!! If any of you wonderful readers have read my Sherlock book as well, you'll know who she is. If not, refer to the screenshots above, they'll explain everything (or go read the Sherlock x Sister!Reader oneshots).

(The website I use to make my OCs is called Charahub, and I HIGHLY recommend using it! It's so helpful, especially when you're like me and you come up with too many characters to keep track of. They give you the option of creating groups where you can herd certain character-chans, and there are a lot of questions about the character that really make you think!)

Requested by Silverfeather__, thank you, love!

Again, sorry if I get the description/layout of the Avenger's tower wrong. I don't know what it's like; I haven't seen the Age of Ultron yet. Or whichever one has the Avenger's tower in it, lol!

(you are Olivia in this shot, but it's not from your POV, just a heads up. also pretend she has the same deducing ability as Sherlock—usually she doesn't but for this shot she/you will :) )

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Thor landed in front of the Avengers tower, letting Mjolnir hang at his side with the leather strap around his wrist. He was exhausted, having just come back from a battle in another realm, and now he had to try and talk to Loki. He walked into the building, but he could tell something was different. A different voice was ringing amongst the conversations of the usual ones he was used to. He took the elevator to the main living room, and entered to see the rest of the Avengers gathered on and around one of the couches.

"Greetings, friends!" he bellowed, turning every head towards him. "Are we all gathered here for a reason? A celebration, perhaps?"

"We're welcoming the newest addition to the Avengers, Thor," Natasha said, beckoning him over. "Come and say hi; introduce yourself," she murmured as he walked over to the assassin. Everyone else had gone back to their conversations, so her words were camouflaged. 

Now that Thor was closer to the group of superheroes, he could see the new arrival: a young woman with long, wavy black hair and sparkling blue eyes.

"Ah, is this the new arrival?" Thor turned to the lady in question with a charming smile on his face.

"Yes, Olivia, hello," she said with a grin, her British accent ringing in Thor's ears. His face immediately changed from 'pleased to meet you' to 'what did I just hear?', and he simply stood there, dumbfounded.

Some of the others started to chuckle at the god's expression, including Olivia. She held her hand out for him to shake, taking a step closer to him.

"Pleased to make your acquaintance, Thor Odinson." 

Thor unfroze upon hearing his name (the mention of it also confused a few other members of the Avengers), and took Olivia's hand and placed a kiss on her knuckles. "The pleasure is all mine...Pray tell..how did you know my name? Did one of my companions tell you?"

Olivia shook her head. "No. I figured who you were. Not many people go around dressed like the god of thunder. Plus I know my mythology; these things do not often catch me off guard."

Thor decided to dismiss the oddity and turned the subject around. "Have you been brought here for a specific purpose, Lady Olivia? A special talent, perhaps?"

Olivia laughed. "I suppose you could call it that. Oh, no thank you, Mr. Stark," she said as Tony offered her a glass of champagne, who shrugged and kept it for himself. "I notice things, Mr. Odinson. Things people usually don't notice."

"Thor, please," he said, now quite intrigued by this woman.

"Thor, then. You wonder how I do it? What exactly it is that I do?" She looked around the room and made eye contact with almost everyone else. "You all do?"

A few of the others nodded, and she continued. "I can see things in others no one else can. Sometimes I miss a few things, but for the most part my deductions are flawless."

"'Deductions?'" Bruce asked, confused. "Like a detective?"

Olivia turned to him and simply said: "Exactly. I'm not a detective as a career necessarily, but I do assist my elder brother. He is the world's only consulting detective, meaning when the police are at their wit's end—which is always—they go to him."

"So if you're...'deducing' things all the time, you must be pretty smart," Tony concluded with a prideful smirk. 

Olivia turned to him once again. "Yes, although not nearly as much as my genius brothers. We three are all intelligent in different ways. Mycroft, my eldest brother at seventeen years my senior, is the smartest of us all; made Sherlock to believe he was an idiot as a child, when in reality they just had not been around other normal children to know that they were a little too special for this world. Sherlock is only ten years older than me, and honestly he and I are quite close, although he is not very good with feelings. People often mistake him for a psychopath; he is a high-functioning sociopath. If he ever feels emotions then he rarely shows them." She rose from her seat and began to walk around. All eyes were on her, listening intently. "Even with our differences however, we three are all the same. Quite intelligent. We see things about others that they probably didn't notice themselves."

"How so?" asked Steve, "what do you see about other people?"

Olivia spun around to face him, her eyes instantly locking with his. "I can see that you were in a hurry to get here once you'd found out I'd arrived. So much of a hurry, in fact, that you deserted the idea of getting something to eat. It's only 13:30 and I only just got here half an hour ago, so you had plenty of time to eat before I came. To the common man it may just seem like maybe you weren't hungry, or you simply forgot about lunch, but I can see that's not the case. I mean, look at you. You're built like an ox, your metabolism must only be as monstrous as your biceps. You wouldn't easily miss a meal without paying for it later. So why would you skip lunch, Steve? I'm not quite as good as my brothers; they would know immediately, but this is as far as my intellect will carry me. Please, Mr. Rogers, tell me: why did you skip lunch?"

Everyone stared at Olivia, until she noticed they weren't quite staring at her—they were gazing at someone behind her. She turned around to see her brother, Sherlock Holmes.

"Hello there, brother mine," she greeted him with a smile, which he returned briefly. 

"Hello, Livvy. As for your friend here, I think I can answer as to why he hasn't eaten his favorite meal of the day."

Steve shifted in his seat, half perplexed as to how this man knew that lunch was his favorite meal and half amazed at the fact that he did. 

"You see, sister dear, you've missed the most important and frankly obvious clue in the evidence here."

"Oh? How do you mean, Sherlock?"

"Well, I'd love to just tell you, but I know you want to learn, so I'll resolve myself to simple assistance. Take a look at his belt buckle. What do you see?"

"The prong is set next to a very worn hole, meaning he sets it to that one every time, and this new setting is unusual. It's a tighter setting. But...what does that have to do wi-"

"You said it yourself earlier," Sherlock interrupted Olivia. "Think back to what you were saying in your earlier deductions."

Olivia turned her gaze to the floor, then to the ceiling, then to Sherlock, her bright blue eyes (which eerily matched Sherlock's) constantly flitting around like she was trying to follow a fly. "What did I say? What did I say? What did I say?" she muttered over and over, then gasped and looked up into her brother's eyes. "I said he was built like an ox!"

"Right!" Sherlock cheered, "He's been getting smaller around the waist, but doesn't want to buy new pants or belt. And no one just gets that kind of body for no reason. Well, except for the Super Soldier, but even if you're just handed a physique like that, you must maintain it. He gets in a lot of exercise. You also mentioned he is likely to have an incredible metabolism. And what do people who regularly exercise and have a high metabolism consume?" he said, trying to lead his little sister into finding out the answer for herself.

"Protein! Lots of it!" she cried, like a kid at Christmas; so excited and full of energy.

"Exactly. Steven Rogers had a protein shake for his midday meal so that he wouldn't go completely hungry, but so that he could also go to the gym as he always does."

Bruce, Tony, Clint, Natasha, Thor, and Steve all stared at the Holmes siblings in dazed shock, but soon began to clap and cheer. 

"That was amazing!" cried the Super Soldier, and the others agreed wholeheartedly.

A few minutes later, when most of the conversation was back to normal, Olivia turned to her elder brother.

"Sherlock, I have one more question." He nodded, signaling for her to continue. "How did you know it was a shake specifically that Steve had for lunch?"

"Didn't you see it when you looked at his belt?" Olivia shook her head. "Powder," the detective went on, "protein powder, the kind you shake up with water. He accidentally spilt some on himself and didn't notice. Look closely next time, sister mine. These things are very easy to spot if you can find them. Sniff them out. You'll get the hang of it eventually."

Olivia nodded, and just then Sherlock's phone began to ring. He picked up, and Olivia could hear the voice of Mycroft on the other end. She rolled her eyes and gave Sherlock a quick hug, which he returned. "Go on and attend to our brother, I'll keep my eye on my new friends," she said with a wink.

The high-functioning sociopath smiled and kissed his sister on the cheek before walking briskly to the elevator, and left.

Thor tapped Olivia on the shoulder, and she turned to face him with a smile. "I must say, Lady Olivia, that your display of wits was quite impressive," he boomed, making her grin even wider. "My younger brother Loki would get along with you quite well, I assume."

Olivia nodded, "I'll have to meet him sometime. But in the meantime, let's get to know each other a little better..."