Episode two of Sherlock, The Lying Detective, threw us an earth-shattering twist – Sherlock and Mycroft have a sister. And her name is Euros.

We’ve already told you about the Greek mythological origins of the name – Euros was the god of the ill-fated East Wind, which ties in very nicely with the allusions to that we spotted back in series three of Sherlock. They in turn are the result of Sherlock creators Mark Gatiss and Steven Moffat taking a quote from the original stories, about the advent of the First World War – “there’s an east wind coming” – and turning it into a reference about Sherlock’s wayward sister, and the havoc she is presumably set to wreak.

But did the quote inspire Gatiss and Moffat to choose the name, or did the name come first and remind them of the quote? There’s good evidence that it may be the latter. You see, Gatiss and Moffat have actually worked with a real-life Euros…

Euros Lyn is an award-winning Welsh director who worked on several episodes of Doctor Who, including two written by Gatiss – The Unquiet Dead (2005) and The Idiot’s Lantern (2006) – and two penned by Moffat, Silence in the Library and Forest of the Dead (2008).

What’s more, Lyn also directed the second ever episode of – you guessed it – Sherlock, back in 2009.

Gatiss and Moffat say they plotted out the twists and turns of series four (and five, should it happen) while filming season three finale His Last Vow in late 2013. It's that episode that features those references to the east wind, so it's quite possible they were slipped in at the last minute.

Certainly, it doesn't seem a huge stretch to suggest that the name Euros may have stuck in their heads from years earlier and that the seeds of Sherlock's sister may have planted even before the series began.

Sherlock series four concludes on Sunday 15th January at 9pm