Notes: I think Sherlock, John and Mary are great together as they are - John and Mary as a couple and Sherlock as their friend - and I'm sure Sherlock is pretty happy with this situation because he wouldn't be interested in anything else anyway.
But just because something is a little out of character, doesn't mean, that it's not fun to read and write these stories anyway..
My stories are a like public television: they fade out when things get serious...
Story: Sherlock knew, that his brother was there when he got out of the cab in front of his house. He sighed. He had a case going on, that he wanted to spend the day on. He had done some internet research in the morning before he had called the cab to get home and now he wanted to start sorting his thoughts by spreading notes and pictures over the living room wall like he always did. He had lost enough time by having to go home first, he didn't want to lose any more time by having to talk to his brother. Even more so when he was pretty sure he knew what Mycroft wanted to talk about.
He walked up the stairs. Mycroft sat on the sofa holding his umbrella and smiling at Sherlock when he came in.
"Where have you been?" he asked.
"Good morning to you, too! On a case" was Sherlock's answer.
Mycroft snorted. "What case?"
"Nothing for you to be concerned about!" Sherlock answered.
"How are John and Mary?"
"Well, I assume. What do you want?"
"Can't I visit my brother just to see how he is doing?"
"Since when do you care how I - or anybody for that matter- is doing?
"I don't, but you do, don't you? You care for Mary and John Watson, Sherlock. You have done since you met them. You have really made friends, what I never thought was wise, because it gives you a weakness, but what do I know..."
"Yes, what do you know!"
"I had almost become willing to accept that your friendship with John had some advantages. You really seemed content and almost happy on occasions since you met him. Of course Mary put you in the awkward situation to become the third wheel, but you seemed to handle even that quite well in the end. You had two friends now instead of one."
"I had... And still have, for that matter!"
"Friends... Yes..." Mycroft laughed. "You yourself can better estimate, if that's still the appropriate word." He got serious again. "You do more than 'care for them' now, don't you? If this friendship is so important to you, why are you ruining it?"
"What do you mean?"
"How long do you think, this will work out? And what do you think is going to happen when it doesn't work out anymore? Do you think you can go back to being mates again who chase criminals together? Your friendship is already over and whatever this is will be over soon as well and then you are on your own again. I wouldn't think that would be the worst, but I assume you don't share this opinion."
"Mycroft, I didn't ask for your opinion! How closely exactly are you monitoring us?"
"Us... There is already an 'us'?" Mycroft shook his head in desperation. "Close enough to know how often you spend the night there. At first I thought you were having a really exciting case that you mused about with your two helpers until it was to late to get home. But after you went there very late in the evening several times during the last weeks and returned in the morning I am pretty sure you don't do that to sleep on their couch when you have a perfectly comfortable bed here. Then I thought you and John had finally crossed the line and there would be a divorce and a happy announcement soon. But it became obvious, that Mary is at home, when you visit."
"Mycroft, stay out of this!"
"YOU should have stayed out of this!"
"But I haven't! And that's none of your business!"
"Unwise, brother mine! Becoming lovers has ruined many friendships. For people that are much more skilled in these things than you are. And I would assume this special situation - whatever it is - doesn't exactly make things easier. Tell me about your arrangement: Does Mary let you and John spend the night together and sleeps on the couch? Or is she watching...? She seems pretty open minded..."
"So you don't have bugs or cameras in their bedroom yet? How nice of you!" Sherlock snapped. "Do you really want me to tell you any details? I would love to do that just to see how you lose it the second I mention that someone loosens his tie!"
Mycroft got up. "Contrary to what you might think, I really want you to be happy. Whatever that means for you... I really liked the thought, that you might have found friends. And now... You are going to loose them again." He went to the door.
"I'm not going to loose anything!" Sherlock said. "This has evolved over a long time. We didn't start it lightheaded. It's not that we just started to ravage each other one night. It's not like that!"
Mycroft turned around again. "Are you... In love...?"
"Yes... I think, I am... With both of them..."
Mycroft snorted again and shook his head. "Look at you: over 30 years of almost nothing, and then you start with a threesome..." He turned around and left.
Sherlock stared annoyed at the door long after Mycroft had left.