However, it wasn’t the brother everyone had been expecting - instead, we were introduced to Euros, their sister.
Actress Sian Brooke is playing the mysterious addition to the cast, and while we’ll have to wait to find out more about the character, we can tell you about the actress herself.
So, in the immortal words of Nikki Grahame, who is she? Get to know her with our nine facts in 90 seconds…
1. This is far from her first TV role
Viewers might recognise Sian from - *deep breath* - ‘Cape Wrath’, ‘Meadowlands’, ‘Doc Martin’, ‘New Tricks’, ‘Vexed’, ‘Silk’, ‘Garrow’s Law’ or ‘Foyle’s War’.
2. Sian’s most recent TV role was in ‘Not Safe for Work’
The actress played Martine in the Channel 4 comedy.
3. Her stage credits are also impressive
Sian has appeared in a number of plays, including various Shakespeare productions. In 2009, she played the female lead in the Royal Shakespeare Company’s ‘Romeo And Juliet’, and has also starred in ‘King Lear’ and ‘A Midsummer Night’s Dream’.
4. She’s already worked with Benedict Cumberbatch
Yes, on another Shakespeare project. Sian starred opposite the actor, playing Ophelia, in the Barbican’s 2016 production of ‘Hamlet’.
5. Sian was also in the first episode of series four
In the final moments of Sunday’s show, it was revealed that the woman John Watson nearly cheated on his wife with was actually Euros in disguise.
6. Her real surname is Phillips
She changed it to avoid confusion with the prolific Welsh actress.
7. Her screen debut was in a Disney series
She starred as Krista in the 2002 mini-series ‘Dinotopia’.
8. She’s good at keeping secrets
We are, admittedly, making an assumption here but Sian was first spotted on the ‘Sherlock’ set way back in June last year. This means she’s known all about the series’ biggest secret - so far - for months. And we haven’t heard a peep about it.
9. She could have a huge part to play in the much-hyped series finale
Sunday’s (14 January) episode is so epic that it’s being simulcast in cinemas up and down the UK. And judging from comments Mark Gatiss and Steven Moffat have made, carnage is in store.
Given that the closing scenes of last week’s show saw Euros firing a gun at John Watson, we’re going to hazard a guess, and say that she’ll probably have a lot of involvement in the events that occur.