This was something I wrote a while ago in reply to a request on tumblr. I just found it again and edited it a tiny bit and thought I'd share!
A knock on the door was what made John look up from his laptop. He was busy blogging their latest case - which was what he told Sherlock everytime he interrupted him proclaiming he was bored.
"Sherlock?" Mrs Hudson began.
Sherlock sighed dramatically from his place lounging on the couch. "What is it Mrs Hudson?"
"There's someone here for you." With that she stepped aside and Mycroft and Lestrade walked in. "Do you boys want tea?"
Sherlock mumbled a 'no', his voice muffled by the arm of the couch he has his face buried in whilst John shook his head at his boyfriend's childish behaviour and thanked Mrs Hudson for the tea.
Mycroft took a seat in Sherlock's chair, much to Sherlock's annoyance, and wiped some invisible dust off his shoulder. "Bored again I see, brother."
"Turning up unwelcome and unannounced again I see, brother." Sherlock spoke without lifting his head from the couch arm.
"Now, now, don't be bitter. You know Mummy doesn't like it."
Sherlock tutted. "Mummy isn't here."
"And it's a good job she isn't. You know she wouldn't approve of your behaviour." Mycroft stared at his brother accusingly, as though that would encourage him to stop complaining.
Mrs Hudson came back in giving Lestrade and John their tea, both of which thanking her quietly as they watched Mycroft and Lestrade squabble.
"So what are you here for Mycroft? Lestrade?" John spoke politely while simultaneously trying to hurry the session along.
"Well, I'm glad someone asked-" Mycroft smiled, though it was clearly one he employed for civil purposes and not friendly ones.
Sherlock made an unattractive snorting noise. "I wouldn't bother, John. It's probably boring. As is everything else right now. Show yourself out, Mycroft. And you Geoff."
"It's Greg." Mycroft retorts with an irritated huff.
Sherlock lifts his head and barks out a laugh. "You'd know. You scream it out often enough-"
"Sherlock!" John scorns, nearly spilling his tea.
Sherlock looks at John condescendingly. "Don't act like it's not obvious."
John blushes as Mycroft and Lestrade look at him questioningly. He glances at the two before turning to the consulting detective and lowering his voice. "But- but that still doesn't mean you have to declare it."
With a roll of his eyes, Sherlock sat up. "Either way, I don't care for your relationship." He stood up and faced Lestrade. "Don't worry, there will be no protective brother routine from me."
It took a few seconds for Mycroft to recover but when he did, he narrowed his eyes at Sherlock. "And you don't have a romantic and sexual relationship with John?"
Sherlock shrugged and made his way over to a very flustered John. "I never said that."
With that, Sherlock sat on John's lap who only had half a second to quickly put down his tea. He perched awkwardly under the gaze of his brother (and his brother's boyfriend) before melting into John's embrace. His legs hung over the arm of the chair and he nuzzled his head into John's neck - which was still red and warm - as John slowly wrapped his arms around his boyfriend's lanky form.
There was a pregnant silence in the room until Lestrade spoke up. "You know, I didn't actually think anything was going on."