This scene always breaks my heart. As much as Sherlock and Mycroft don’t really have much love lost between them, they are also close in a way - see the moment when Mycroft gives him the cigarette for Christmas, for example. Sherlock isn’t close to many people, and he is almost forced to trust the ones he is close to. If he had the option, he’d probably only trust John - but Mycroft always knows his secrets even if Sherlock would prefer to keep them private. In this moment, as they’re discussing an area that Sherlock is inexperienced in and as such made vulnerable by, Mycroft betrays him by revealing that weak spot. Look at Sherlock’s face - that’s not anger or annoyance. That’s pure, unbridled hurt from a cut only Mycroft had the power to give him.
Ah, but look what happened earlier in that scene:
J: Here to see the Queen?
S: Oh, apparently yes.
Mycroft looks hurt there. He has to take a moment to regather his composure before speaking. Both have sharp wits and old scores to settle - when the opportune moment arose, it is unsurprising that Mycroft retaliated. In the language of the playground - which is, sort of, the level our boys like to operate on - Sherlock started it.
Yes, they are cruel to each other. Very cruel. But they are family, and so can get away with it. Each gives as good as he gets. But when it matters - really matters - they are there for each other.
John may be Sherlock’s best - and only - friend, but there are still times when he needs his big brother more. When Irene sends Sherlock her phone on Christmas Eve, this is shown through a very clear visual metaphor. Sherlock retreats to his room and calls Mycroft then, when John tries to come in to ask if he’s all right, Sherlock shuts the door in his face.
Throughout Scandal, Sherlock turns to his brother when he needs him. And it’s not an entirely utilitarian need either - just look at the morgue scene. Sherlock turns to his brother for emotional support there - “Do you ever think there’s something wrong with us?” - and Mycroft gives it as best as he can.
Sherlock’s deep, intrinsic trust in his brother is something I hope we will see vindicated in Series Three when - as I hope and believe - it is revealed that Sherlock has turned to his brother for help once again. Sherlock is not forced to trust his brother - far from it - he trusts him for much more than the simple things that makes Mycroft useful.
They may have a very bitter, brittle, distanced brotherly relationship - but it is a bond that has tied them together in their odd, Holmesian way.