“Initially he wanted to be a pirate” is such a lovely line, and the face that he makes - that half-smile dissolving into sadness as he remembers Sherlock’s pirate days - he looks utterly haunted by his past. This is the first clear window we have into the Holmes brothers’ childhood, and it breaks my heart every time. But why did Mycroft bring it up? For an answer to that, we may have to look at the context of the scene as a whole.
The whole cafe scene from Scandal is a bit of a foxy one, as the whole conversation between Mycroft and John can be read as being a test. If many Reichenbach theories prove correct, mine included, then, by this scene, the brothers are already laying the groundwork for their final battle with Moriarty and are trying to decide whether or not they can bring John into the fold.
Consider the following: That Sherlock wanted to be a pirate when he was a boy is one of the few indisputable facts we have been given regarding Sherlock’s childhood, and from the most reliable of sources. This is also a fact that, it would appear, Jim Moriarty is not privy to.
As Mycroft was, allegedly, the one supplying Moriarty with the information to destroy his brother, one would think Sherlock’s boyhood fascination with piracy might have been brought up again. Instead, Moriarty’s playing with another familiar childhood trope - fairytales. He’s leaving the collected Grimm lying around, cooking gingerbread men, carving red apples, re-enacting a murderous version of Hansel and Gretel - he’s really pushing the theme throughout Reichenbach. But why? It’s possible that fairytales hold a fascination for Jim Moriarty himself, but it is equally possible that Mycroft led Moriarty to believe that Sherlock’s childhood fascination lay in the area.
I fully believe that Mycroft was working alongside his brother throughout the events of Reichenbach, but he will still have had to tell Moriarty something during their little interrogations. Moriarty is a genius; he would smell a lie before it was even spoken. But Mycroft is a genius too, and he could make a half-truth seem plausible.
Therefore, that Mycroft mentioned Sherlock’s love of pirates to John may have been him throwing out a small clue in the hope that he picks up on it.
Before closing however, I must say, despite this being my theory, I’m not wholly convinced of it. I do think Mycroft was just reminiscing about the childhood he and Sherlock had shared. He does have something of a soft-spot for his brother, after all, and it can’t be often he gets a chance to talk about him. Oh, I so hope we learn more about the Holmes Brothers’ past next series!
