not a dragon for you to slay.

Mycroft has to explain that no, you can't marry your sibling. Sherlock is severely disappointed by this, so Mycroft reassures him by telling him that while people divorce their spouses all the time he'll always be his big brother no matter what happens.

Many years later, Mycroft assumes that his younger brother has no memory of this episode, but Sherlock remembers it perfectly.

One day Sherlock and Molly get pretty drunk together. Molly starts talking about marriage, wondering if she will ever get married, and tells Sherlock that he must never have thought about getting married. He replies that he once wanted to marry someone : his brother.

He figures that it’s of no importance, because Mycroft is the government and controls everything. That’s all one needs to know.

And it makes me think about how when Mycroft calls from Les Miz and Sherlock says but you made a promise. How when Mycroft tries to talk him out of protecting Mary, he says he made a vow and Mycroft accepts it.

And how in TAB Mycroft basically promises that he will always be there, and Sherlock does not make a promise that he knows he cannot keep.