There... is no explanation for this one really. Only a surprisingly succinct summary: Snowball fight at the estate. Mycroft is not amused. All recognisable content belongs to its respective owners.
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It was John who started it. Later, they would all blame Sherlock, but it was always John who started it. This time, the doctor chose to lobb a perfectly aimed snowball straight into the back of the most dangerous man he'd ever meet, only to a mere second later toss one straight into the dark curls of said man's laughing little brother, leaving both brothers spluttering and making snowballs of their own, mostly tossing them onto each other, to the amusement of not only the doctor, but their father, who joined in somewhere along the line.
They were out until they were all splattered with snow and both brothers desperately (though they'd both stubbornly deny it) needed to warm up their hands, as they had foregone truly steady gloves, while their father and John had somewhat been more sensible (as usual).
Their mother, as well, found the sight of her sulky and wet sons somewehat amusing, but she restricted herself to having the servants draw baths and heat towels for all four of them, looking very pleased with herself as they all an hour later were relaxing together before the grand fireplace, actually eating and enjoying her cookies for once. No one was even arguing, since both brothers were far too tired to bother.
The evening progressed peacefully, with Mycroft contentedly reading a very heavy book which at least John thought far too serious for a relaxing evening in. Sherlock, worn out from stubbornly trying to hit his brother with as many snowballs as was humanly possible, seemed just as pleased with lying on the ottoman before the fire, getting warmed up again. He rested his head in John's lap as he liked to do, eyes closed which only enhanced the impression of a lazy cat.
John spoke with the two strange brothers' parents, complimenting Mrs Holmes on the Christmas cookies and slowly running his fingers through Sherlock's hair. The two older Holmeses sat together on the sofa, holding hands and looking exceptionally pleased with the peaceful evening. Mrs Holmes in particular seemed very happy with her sons actually sitting still for once and not whining.
All in all, it was a perfectly pleasant evening, and if John Watson winked at her when neither of the brothers were looking, she wasn't going to tell, now was she?