In orginal canon, Holmes mentions that he is decended from "country squires". In the 19th century and earlier, squires would own a manor house and the land on which the town around it was situated. The squire was responsible for the management of the town.
William Baring-Gould, who wrote the Sherlock Holmes biography, speculated that there must have been a brother older than Mycroft who had inherited the family's country seat.
So the idea of a third brother (or sibling) is semi-canonical, if one wants to accept Baring-Gould's ideas as part of canon. Since Moffat and Gatiss borrowed Sherlock's full name from the Baring-Gould biography, I assume they are borrowing the idea of a third sibling also although it remains to be seen if the above quoted line leads to the addition of a new character or was simply a throw-away line.

Freeman told Alan Carr at the recording of tomorrow night’s Chatty Man episode: “Mark Gatiss may beat me up, but there is an idea for this one-off special that’s such a good idea, and as I was listening to it I thought ‘we’ve just got to do this’. And I don’t know when we are going to be able to do it, unfortunately.”He added: “It’s a fantastic, really mouth-watering idea. But I really don’t know when we are going to get to do it.”Last week the actor revealed that it had been intended to film the new series later this year: “I thought the plan was to do it in the autumn but apparently that’s fallen away.” 

I was always under the impression that the name "William Sherlock Scott" came from the Baring-Gould "biography" of Sherlock Holmes published in the early 1960's. Same source as the idea of a third Holmes brother.

Ok I never post to these things but I decided to do so here.
I knew that Mary would not be killed off at the end of series 3 because it would have made very little sense to do so after having spent a whole episode, when there are only  3 the begin with, on the wedding. I mean why 'waste" a whole episode on a marriage that lasts a month?  And they didn't have to kill her if they really wanted to get rid of the character--Mary's lying  to John about her past/shooting Sherlock could have ended her right there. In fact, Moffat once said in an interview that he wanted to look at the idea of Sherlock and John living apart  but didn't  know if he would have John get married. He obviously had his reasons for deciding to do that.
Sorry folks, but the baby is the issue here. That's because the idea of the pregnancy/baby is so far out of canon that to bring it into the story and then just have it go away (loss of pregnancy/baby death/child living elsewhere) just for the sake of canon is pointless.  Add to that, the fact that they throw canon out the window all the time. Now Moffat and Gatiss have been known to introduce  pointless plot developments into their stories (Molly's engagement, for example), but I don't think this is one of them. This all has to do with the fact that Moffat is very fond of the idea of family. The idea of "family" is all over his past projects--Coupling,  Jekyll, even Doctor Who (Amy and Rory were the first married couple to be The Doctor's companions. They even had a child--although that didn't work out like it usually does, but still...). And you couldn't miss the family bonding theme in this 3rd Sherlock series.  Anyway, what Moffat is doing here is something  that is sometimes done in genre TV--giving a/the protagonist a surrogate family. Through John and Mary, he is giving Sherlock a kind of family (i.e., couple/child) that he will never have on his own. This has nothing to do with what is or is not canon and everything to do with an id