William S. Baring-Gould was a smart man. A much smarter man than I.
Baring-Gould, for example, counseled that we not confuse Montague Place, mentioned in "The Adventure of the Copper Beeches," with Montague Street, the place where Sherlock Holmes started his detective career. They are definitely separate streets, but both bordering the same block, and definitely intersecting with each other were it not for the interference of Russell Square. (Using Google Maps and all its marvelous views, you can pretty much fly around the area like the hand-in-hand Holmes and Watson aerial trip Holmes once suggested. Not taking off the roofs, of course.)
Suffice it to say, they're pretty damn close to each other.
So when one considers that Sherlock Holmes started his career from a street named Montague in that neighborhood, and that Violet Hunter of "Copper Beeches" started her career from a street named Montague in that same neighborhood, you might want to start disregarding Baring-Gould's sage advice and go, "Hmmmm."
A number of writers have theorized that Violet Hunter was a sister or half-sister to Sherlock Holmes, and when you consider the odd coincidence of the streets named Montague, the theory starts to gain some weight. Where does one often start one's career? Out of the family's residence. The kind of place you'd split rent with a room-mate to get out of and live on your own. Or stay in between governess jobs that you needed to ask an elder brother's advice about.
And speaking of elder brothers -- who do you think might have been the primary resident of these Montague quarters? We are told Mycroft Holmes "lodges in Pall Mall" -- a little over a mile away -- when he is first introduced, but might he not have started in Montague as well, later moving to Pall Mall and keeping the rooms open for family in town? (We know from "The Empty House" that he kept Holmes's Baker Street rooms going though empty for years, so the concept is not that strange to him.) Or was their widowed mother there? Some aunt or uncle?
I think that this street of Montague coincidence, combined with pre-existing theories of a Holmes sister deserves further exploration. And if, like Baring-Gould, one were to insist that one was, yes, "Montague Street" while the other was "Montague Place," I would remind one that Watson also wrote "Holmes" and "Hunter."
And Holmes's over-dramatic annoyance at being asked to advise on governess jobs when he should be hunting criminals? Oh, that's totally big brother peevishness. But in the end, as Sherlock Holmes said, the Rucastle job was "not a situation which I should like to see a sister of mine apply for."
Shared origins, like that Montague starting point, do count for something.