A/N: Warning! Season 3 spoilers. In fact, Sherlock's present to Mycroft will not make any sense if you have not already viewed some scenes between them in The Empty Hearse. This story is set during His Final Vow, and has some significant spoilers, though you can still understand the story without having viewed that episode. The timeline of events at the Christmas celebration is being played around with a bit.
Christmas with the Holmes family was certainly turning out to be much nicer than John had expected. Reconciliation and a new beginning with his wife was certainly helping John's spirit of holiday cheer. It was also amusing to see Sherlock and Mycroft interacting with their very ordinary parents and trying to be on their best behavior around them. John had a sneaking suspicion that Sherlock had been skulking about outside a time or two with a cigarette, but given his other recent more serious indulgences, plus the holiday season, John was giving it a temporary pass.
And the gift exchange was going quite nicely. John had explained that he and Mary preferred to exchange their own gifts to each other privately, which earned them an indulgent, knowing smile from Mrs. Holmes. But, it was far easier than explaining why John was giving his wife a pen drive with unread information about her previous life as an assassin. Everyone else had given nice presents, ones that were either well-chosen or at least a gift card open-ended enough that the recipient could find something to enjoy.
Sherlock's gift to his parents had been theatre tickets. Sherlock had confided on the drive up that he'd have given his parents three tickets if he had thought he could maneuver it so that Mycroft would have to accompany them. However, since Mycroft had gone the last time, Sherlock's parents would be likely to take Sherlock instead as a thank you for his gift, so it was safer to give them only two. Actually, John wondered why Sherlock hadn't given his brother a ticket to the same performance. It was just the sort of "gift" Sherlock would enjoy, one that would annoy his brother while he could play the innocent.
But now, Sherlock proclaimed, "One more gift to be given. I haven't given my brother his present yet." And he had a broad smile on his face, which was actually what alerted John that something "not good" was about to happen. Any smile that Sherlock had at a sentimental moment like this should have been one of those insincere ones plastered on just to make his parents happy, but Sherlock actually looked rather… jubilant.
As Sherlock stepped out of the room, John glanced over at Mycroft and saw that his posture had switched to high-alert status, but Sherlock's parents looked on with pleased anticipation. Sherlock came back in carrying an unwrapped open-topped box, which he set down on the coffee table.
"I thought this would be just the thing for you, Mycroft. But, I wasn't sure if you'd be able to get one on your own," the last with a raised eyebrow and glittering eyes in Mycroft's direction. As everyone leaned forward wondering what was in the box—except for Mycroft who instinctively leaned away—Sherlock lifted out and held out to his brother… a fish bowl with one big fat goldfish swimming around. "I found a goldfish for you, Mycroft!" declared Sherlock with a challenging air.
The present was much more innocuous than what John had been expecting, so he was surprised to see Mycroft's face turning red.
"Oh, what a nice pet, Sherlock!" Mrs. Holmes exclaimed.
John glanced between the amused Sherlock and Mycroft who was clenching his umbrella handle. Obviously, there was some underlying message that John was not privy to. This needed smoothed over and a distraction provided.
John stepped into the breach, "Yes, always nice to have a pet to keep you from being lonely. Why don't we—" and John cut himself off abruptly as Sherlock's grin became positively gleeful. Mycroft had abruptly turned his head toward John with a gaze that seemed to bore right through him. But on seeing John's confusion, his glare lessened a bit before returning full force at Sherlock.
John wasn't sure what would have happened next if Mrs. Holmes hadn't spoken up. "Well, take it, Mikey! Don't leave your brother just standing there holding your present," and then added in a hissing whisper that carried to all, "and say 'Thank you!'"
With an extremely insincere smile, Mycroft took the fish and said, "Thank you." He put the bowl down rather roughly, creating a tidal wave that sloshed the goldfish around a bit.
"Why don't we go get some more of those wonderful nibbles?" John suggested with a forced grin. "Sherlock?" with a look that indicated John expected to be accompanied.
Once they'd left the room, Sherlock turned to John, "You unknowingly added the perfect touch to my present. It had a much better effect than it would have coming from me."
"Well, I don't exactly know how you did it. But, you obviously managed to give your brother a present that infuriated him, while keeping you in your parents' good favor. Couldn't you behave and give him a nice present, or at least a benign one?" John asked.
"John, what could possibly be more benign than a goldfish?" Sherlock asked with an innocent air. After a moment, and a sideways scolding glance from John, Sherlock relented. "Don't worry. I expect to be in my brother's black books very soon anyway. He can only get so angry. I might as well milk it to its full potential."
And despite John's repeated efforts to query Sherlock as to what trouble he was planning to get into, he couldn't get an answer. That is, until later in the day, when everyone began passing out left and right. But, that is a story that has been told elsewhere.