A/N: Inspired by a prompt from simplyshelbs16: "best friends sibling au."

Sherlock walked into the kitchen to find Molly Hooper sitting at the table, looking as surprised as he felt. She then blushed and he had to admit she was pretty when she did that.

He wouldn't admit, even to himself, that she was pretty all the time.

"Um … hi, Sherlock," she said. She gave him a little wave. "We didn't think you'd be home for an hour."

"We?" he asked, confused, as he set his bookbag down by his usual chair, which was across from the one Molly happened to be sitting in.

"Yeah, 'we,'" Eurus said as she came in from the walk-in pantry, her arms full of bags of crisps. "Molly's helping me pick out a prom dress." She set the bags on top of the fashion magazines already covering most of the table.

Sherlock rolled his eyes. "Are you still planning on asking Gavin?"

"His name is Greg," Eurus said, "which you know perfectly well, and yes – with you lot graduating in May, this might be my last chance to go out with him." She had fancied Sherlock's friend since her freshman year but was only now planning on doing something about it.

He considered warning his friend but decided he wanted to see Eurus get shot down. "I'd wish you luck but not even luck will get Graham to go to the prom with you."

"We'll see about that," Eurus said firmly. "And it's Greg."

"Uh huh." He glanced at Molly. "Please tell me you're not planning on asking Mike Stamford."

"He's my chemistry partner, that's all," Molly said, confused. "Actually … I don't think I'm going."

Sherlock ignored the sudden alarm bells going off in his head. "Why not?"

"Well, no one's asked me."

"You can ask a boy to prom, Molly," Eurus said, smirking. "This is the Twenty-First Century."

Molly blushed. "There, um, isn't anyone I could ask."

Sherlock didn't know it was possible to feel upset and relieved at the same time, yet here he was. "Just go by yourself, then."

"That's what I do at every dance," she muttered. "Then I end up being a wallflower until I leave early. It's my first prom, I wanted it to be different."

Sherlock could almost see the lightbulb turn on over Eurus' head. "Sherlock, why don't you take Molly? You've got to be over Irene by now."

She just had to bring up the ex. "I'm sure Molly doesn't want her best friend's boring brother to take her to the prom."

Both of them turned to Molly, who blushed again. "Um … actually … I'd like that."

Sherlock raised an eyebrow. "Alright."

Neither of them noticed Eurus' delighted fist-pump.

To Sherlock's absolute amazement, Lestrade did accept Eurus' invitation. He admitted to Sherlock later that day that he had been thinking of asking Eurus out but didn't think he was her type. Sherlock just rolled his eyes over the whole thing.

The night of the prom, Sherlock, John, and Greg pulled up in a stretch limo to the curb in front of Mary's house. The girls had decided to meet there to get ready. The three boys got out of the limo and John knocked on the door. Mr. Morstan let them in and Mrs. Morstan beamed at them.

"Oh, look how handsome you boys are!" she said. "The girls are going to be delighted."

Mr. Morstan went to the foot of the stairs and called for the girls. Sherlock held his breath as the three of them descended the stairs. He felt a little silly, then he noticed John and Greg were doing the same thing. He got the vague impression that Mary and Eurus looked nice, but Molly took his breath away. Her hair was curled and pinned up in some complicated hairdo he couldn't begin to describe, but it loved it anyway. She was wearing some sort of lotion that made her skin shimmer faintly, like she was the most adorable fairy. And like a fairy, she was in a ballgown of gold silk that complemented her figure perfectly. Molly looked up at him nervously.

Eurus smirked. "Say something, Sherlock."

"Er … that is … um…" It wasn't often that words failed William Sherlock Scott Holmes, but when they did, it was a spectacular sight. Not knowing what else to do, he grabbed Molly by her bare shoulders, pulled her close, and kissed her softly.

Mr. and Mrs. Morstan didn't mind, it gave them something to talk about at Sherlock and Molly's wedding five years later.