Forget Sherrinford (for now). Forget the fabled third Holmes brother (probably). Because Sherlock and Mycroft have a sister – and her name is Euros.

The massive revelation came at the end of series four episode The Lying Detective – as John Watson’s therapist and the ‘woman on the bus’ he’d almost had an affair with were revealed to be one and the same person – but it was an idea that was conceived far earlier than that, also on a bus, as Sherlock creators Steven Moffat and Mark Gatiss sheltered from the rain during the filming of series three finale His Last Vow...

"The scene where Euros finally unveils herself, we've been sitting on that since we sat talking about it on the production bus... when we were shooting His Last Vow," revealed Moffat at a Q&A following a screening of The Lying Detective. "We've been talking about it ever since then, so it's kind of weird that in few hours time we won't have to kill people to keep the secret."

But in fact, even within the series itself, Moffat and Gatiss have been foreshadowing Euros’s arrival for some time. You see, she’s named after a Greek god (a male one, unfortunately for her) who is synonymous with the ill-fated East Wind.

Euros was said to bring bad luck and, more specifically, rain – which perhaps explains that rather enigmatic promo image of Sherlock and John’s flooded Baker Street flat we picked apart when the series launched. 

It also sheds light on some of the ominous references to the east wind that fans may recall from series three... 

At the end of season finale His Last Vow, as Sherlock and John are saying goodbye for what could be the last time, the detective tells his friend “the east wind takes us all in the end.”

And when supposedly deceased arch-villain Moriarty suddenly rears his head again on screens around the country, asking “Did you miss me?”, Mary demands “How can he be back?", to which John replies, “Well if he is, he'd better wrap up warm. There's an east wind coming.”

There’s a precedent for this in the original Sir Arthur Conan Doyle stories, too. His Last Bow – Sherlock Holmes’s final adventure (chronologically speaking) – takes place in 1914 on the eve of the outbreak of the First World War. In the closing scene, the ageing Holmes warns Dr Watson “there’s an east wind coming,” implying that it will take many lives – and judging by the final scene of The Lying Detective in which Euros aims a gun at John before pulling the trigger, the same may be true of Sherlock’s sister…

Sherlock series 4 episode 2 reviewed