3. Tom Hiddleston

What? We had to at least mention this long-running theory/wish from the Sherlock fandom. Ever since that fateful mention of “the other one”, fans have hoped that the Thor actor would be introduced as the third brother. Mark Gatiss himself added fuel to the flames at last year’s San Diego Comic-Con when he tweeted a photo of himself with Tom Hiddleston. He gave the photo the caption ‘blud’ – a slang term for brother.

It seems unlikely that Hiddleston’s casting would have flown under the radar but, hey, you never know, he could make a surprise appearance in ‘The Final Problem’!

2. Sherrinford’s A Codename

OK. maybe there is no extra brother and Euros is the only secret sibling Mycroft and Sherlock have. In that case, why was Mycroft calling her Sherrinford? Our best guess is that this was the codeword assigned to Euros. Presumably, her casefile is top-secret so that Euros is only referred to as Sherrinford in public.

This would fit in with the MI6 briefing scene at the start of ‘The Six Thatchers’. When Mycroft is presenting the doctored footage of Magnussen’s murder, everyone present is given a codename: names like Porlock and Langdale. If there is no brother, this seems like the most obvious explanation for Sherrinford.

1. It’s Moriarty!

‘The Lying Detective’ seemed to suggest that Euros was behind the ‘Miss Me’ incident at the end of Series Three, as she wrote it in invisible ink on that piece of paper. However, what if that was another red herring from Mofftiss and Moriarty is really out there – set to return in ‘The Final Problem.’

And, what’s more, what if it’s Moriarty who is the secret brother Sherrinford. We don’t know anything at all about Moriarty’s origins – except for the Carl Powers case from ‘The Great Game’ – so it is potentially possible. And, again, he shares the same deductive prowess as Sherlock and Mycroft.

Well, why not? Stranger things have happened…

Have any plausible Holmes brother theories? Comment below! We would love to hear them!