The question is one of the most debated amongst holmesians (well, I’ve still to find a single question which has not been much debated among holesians, sooner or later… we are a debating lot). It arises from some canonical passages which have arisen the suspect that Sherlock and Mycroft could even have had other siblings, either still alive or dead at the time in which the original stories are settled.
We know for certain that Mycroft is seven years elder than Sherlock:
”When I say, therefore, that Mycroft has better powers of observation than I, you may take it that I am speaking the exact and literal truth.”
“Is he your junior?”
“Seven years my senior.”
We also know for certain that the Holmes were not originally from London:
"My ancestors were country squires, who appear to have led much the same life as is natural to their class."
So the first question is: why Mycroft - apparently the eldest Holmes sibling - is not living in the country family estate?
At least three answers are possible, and have been proposed by holmesians over the years:
- The Holmes family was no more as wealthy as it once used to be: the father or grandfather (or another ancestor) of Mycroft and Sherlock Holmes had to sell the estate and the family henceforth moved to London, where both Mycroft and Sherlock were born. In this scenario, they could as well be the only Holmes siblings ever existed.
- The Holmes family still held the country ancestral estate, but for some reason neither Mycroft (which, as the eldest child, would have been entitled to inherit the property), nor Sherlock liked to live there, and both preferred to live and work in London. Also in this scenario, they could as well be the only Holmes siblings ever existed. But such an hypotesis has in time raised further speculation about the reasons of their shared dislike for their ancestral home - and you can imagine that some of the proposed explanations imply some sort of past family tragedy/mystery… (Meyer’s The Seven Percent Solution, for instance, deals with such a supposition…)
- Mycroft, while being elder than Sherlock, is not THE ELDEST Holmes brother: there is at least another male sibling, elder than Mycroft, who inherited the estate. For some reason, he is never mentioned in the Canon. This could simply be ascribed to Sherlock Holmes’ extreme reticence and private nature. A second hypothesis is that this elder brother was not in good terms with his younger sibling(s). A sub-hypothesis of this hypothesis is that this elusive eldest brother and the two Holmes brothers we know (i.e. Mycroft and Sherlock) are the offspring of two different marriages, the younger ones having a different mother than their senior.
So much speculation about the male siblings. Even if it must also be said that NOTHING in the Canon excludes that there could have been further male siblings, either dead in their infancy, or still alive. Families used to be large, in the Victorian Era, and the difference of seven years between Mycroft and Sherlock could as well have been filled with the birth of other children…
And one or more of said children could have been female ones: and here we are, with the second vexata quaestio - did Sherlock Holmes have any sister?
Here again the question arises from a couple of canonical passages:
“I confess that it is not the situation which I should like to see a sister of mine apply for.”
Sherlock Holmes to Violet Hunter
ACD, The Adventure of the Copper Beeches
And yet he would always wind up by muttering that no sister of his should ever have accepted such a situation.
The fact is that Holmes doesn’t add an adjective in the line of “hypothetical”: this could be subtext, or instead it could mean that he actually has (or had) one or more sister(s).
Here again, in the eventuality of him having one or more female siblings, she (or they) could either have been already dead when the story takes place, or have still been alive. And the lack of any further mention on Holmes’ part could be, in the latter case, ascribed to his extremely private nature (less probable is the hypothesis of them not being in good terms, as he shows concern for such a “sister of his”).
So, this is definitely a matter capable of opinion: would you care to share yours?