Sherlock Series 4: 8 Huge Predictions (And Why They Need To Happen)
3. Speaking Of The Past, There's A Third Holmes Sibling
In the third episode of Series Three of Sherlock titled His Last Vow, Mycroft is chatting to someone about Sherlock when he says: “I'm not accustomed to outbursts of brotherly compassion. You know what happened to the other one.” It's a curious reference and one with surprisingly deep implications; too deep to be completely throwaway.
Since it's likely we'll be dipping to the brothers' past in Series Four, it'd be the perfect time to elaborate on this third brother. There are two possibilities: one, that Holmes Number Three is dead, or two (and more likely) Holmes Number Three chose to use his brilliance for criminal gain, and was caught and imprisoned by Mycroft.
A third Holmes brother would be an interesting little twist, and it wouldn't come across as forced or contrived because we've basically already been told about it. If they did happen to be a criminal, perhaps they might be relevant in the present day, too?