Obsession (слэш)

In the aftermath of a Holmes brothers argument Mycroft has to deal with a truth exposed, with a secret come to light and some other things. The results could be catastrophic, and will definitely be unexpected.
From the prompt: "Sherlock is an annoying brother to Mycroft and an insensitive brat to pretty much everyone else, but this time 'round, Greg might want to thank him, as he revealed that Mycroft is interested in him."