After a long hiatus, I am finally going through my inbox and the number of questions about the identify of the third Holmesbrother is phenomenal. People have postulated everyone from Q in James Bond to Moriarty.
I do have my theory about the Third Holmes Brother and it is very simple: he doesn’t exist and I shall explain why it fits perfectly with the themes of His Last Vow.
Moffat and Gatiss love exploiting with our preconceptions to produce amazing plot twists. If there is any episode where facts turned out to be nothing more than smoke and mirrors, it would be His Last Vow.
The Third Holmes Brother is another wonderful piece of deception.