I just realized that this is the one and only time Sherlock actually calls someone (except John in TRF). No matter how big or small or urgent the issue, he always texts, and that too rarely. And yet, on John's wedding day which he pretends is nothing special, he makes a phone call to the one person he pretends to especially hate.
What might we deduce about his heart, then?
The most obvious is that he genuinely, really wanted Mycroft to be there. If it had just been an invitation for the sake of it, he would've texted or maybe just ignored. Calling is his way of showing how much it matters to him.
And why? Because this is a new, alien setting for Sherlock. People aren't his thing. He's visibly nervous, mostly about his actions ruining John's wedding day. That is the one thing he doesn't want, hence the extra effort - the tissues, the kids, the stalker, the lilac, the invitations, the speech, all in place. And yet, he thinks it isn't enough, that the speech is going to be the hardest thing he's ever done, that he'll ruin it all and there'll be people laughing at John for having a 'freak' as his best man and he'll be letting John down, yet again. He's now practically immune to being called 'freak' and stuff, but it's different here - it'll be aimed at John! That is what he's scared of. Note that he doesn't fear being ridiculed, or being left all alone, or anything else, just letting John down - that is what scares him. And so, like in all his most vulnerable, difficult moments, whom does he turn to?
Why? Because for all these years, as the curly-haired kid and now as the hat-detective, it has always been Mycroft protecting him. So to turn to him now is an instinct. Sherlock knows that if there is one person in the world who knows him inside out, understands him, wouldn't judge him, and will ALWAYS be there for him - it's his brother. No wonder Mycroft is the only person with whom Sherlock lets his guard down - he's always the baby brother asking questions about the goldfish world around them, not the all-knowing detective ridiculed both for knowing and not knowing.
"You ever wonder if there's something wrong with us?"
Sherlock wouldn't ever dare ask anyone else such a question, because the standard response would be exclamation at him showing 'sentiment'.
"Caring is not an advantage, Sherlock."
Mycroft sees right through the question and his response is, like always, to protect his little brother. Mrs Hudson thinks otherwise, but Mycroft has always known how "he's more emotional" and therefore more vulnerable to hurt. With all his words and actions, Mycroft's only endeavouring to protect Sherlock from that, always. And little brother knows it.
By far, theirs has got to be the most beautiful relationship in the show.