For whatever reason, the government is in some kind of an uproar and Mycroft basically can't trust any of his normal resources at the moment. He has to hide out somewhere he can't get back stabbed while he sniffs out spies/traitors and gets things sorted out. To everyone's bafflement, he chooses his little brother's place (he probably says it's because hiding with family is so obvious that no one would expect it from him, but it's most likely because he wants to keep a more careful eye now that he's temporarily out of influence and can't stay in the loop anymore).

Sherlock is fairly pissy about it, and if it were up to him Mycroft would have been booted the minute he arrived. Luckily, John is there to smooth things over. Just a fluffy fic of John mediating between the brothers and making Mycroft feel comfortable and at home, since Mycroft would never actually admit how much the loss of power/feelings of usefulness/betrayal affected him. No ships please.