Neither one of them showed up. No matter what old scores or rivalries they should have been there. Who misses their siblings wedding?
Harry had made it about choosing Sherlock or Mary. Told them you can only love one person. That John was only marrying Mary because he was ashamed of his love for Sherlock. It didn't matter that Sherlock and Mary were in love with each other, too. And that they had suggested the arrangement. To Harry John had to meet her standards for his relationships. And he wouldn't--couldn't--let anyone control him anymore. So Harry didn't come.
Mycroft disproved of emotional weakness. It was bad enough that Sherlock had friends. They left him open to attack and made more work for Mycroft. Sherlock's vulnerabilities always became his problems. And when he inevitably lost them Mycroft would be the one to put Sherlock back together again. Just like Redbeard. Mycroft pushed the dark memories from his mind. He had better things to do then watch his brother make this mistake. Like try to work off those stubborn five pounds Sherlock liked to tease him about. Weddings were a ridiculous tradition. He refused to go.
It was a lovely wedding. The bride was radiant, the groom happy, and the “best man” content. (And surprisingly not bored. Attempted murder after all.) If only they weren't missing two very important people.