Erosion
(Takes place after the beginning of Season 4, Episode 3. Spoilers. This is not what happened, but it easily could have been.)
John and Sherlock are laughing when Lestrade shows up with his warrant. They don't understand. He explains.
Accessories to murder. Whose murder?
Your brother, Sherlock. Mycroft. He was murdered last night.
Utter stillness. Not a quiver. Frozen in shock.
It seems, Lestrade explains, that someone disabled all his security, despite knowing he had many enemies. Last night, one of them caught up with him. And he had no way to defend himself.
Sherlock goes quietly. Later, he'll break out and try to solve the case, but right then he goes quietly. There is no big brother to interfere.
When Lestrade interviews him, John tries to explain why they had to do it. You don't understand, he says. We had to make Mycroft tell us about his sister.
You couldn't look her up in public records?
We—well . . .
Or ask his parents?
Ah . . .
Did you, in fact, ever ask Mycroft if he had a sister?
I asked if he had another brother.
And did you explain what was going on to him? That's what you expected to happen, isn't it? He'd come to the flat and sit nicely while you explained what was going on and then he'd help you. So why not just ask?
It made sense at the time.
It made sense at the time to pointlessly terrify and endanger a man (admittedly, a sometimes annoying man) who has never done anything but his best to preserve his brother's life, your life, and the wellbeing of our beloved country?
It made sense at the time, John repeats. He can think of nothing else. The goodness in his heart has been eroded, and he shies away from looking at what has taken its place.
We thought it was funny.