How old are Sherlock and Mycroft Holmes in the BBC Sherlock TV-Series? Do they have other brothers and/or sisters?
For there is no reference in the show, one can only assume that they are the same age as the actors:
If you are going with the canon, however:
As pointed out before the show makes a reference to "another one". So there is a possibilty that there is another Holmes brother. But at the end of S3, we can only guess.
According to the novels A.C.D. wrote, there is no other brother mentioned. But a possible explanation that there might be one, is given here:
Since the series takes place in the 21th century, there is no need for the eldest brother to manage the house. This only leads us back to guessing ;)
Though there has never been a direct mention about Sherlock's or Mycroft's age, but in the episode The Empty Hearse, there is a slight reference.
Regarding the third Holmes sibling, nothing has been made clear yet. Hope the coming seasons will reveal the mystery.
Source (Transcript) : Ariane DeVere