You do realise you pulled that rule out of your hat right. The title can be whatever the Hell the author, editor and/or publisher think good and however abstract they want.
They choose a title base on what they wanted to imply, theres a lot of LN/WN that has the same title structure, its a famous structure since it's a really straightforward way to describe your work, you can take it as face value. The title here literally means and implies that the brother knows her sister is into yuri so i will take that as a fact.
It certainly can mean, and be read as, that. But it is under no obligation whatsoever to actually mean it at least in such straightforward sense.
To illustrate the point invoking the herein oft-mentioned but sadly unhosted It's Not My Fault I'm Not Popular should be enough as the title is patently false on multiple counts and for reasons that change over the course of the story...
But thats the point of that manga, the protagonist denies that it's not her fault that she's not famous even though it clearly is. The author wants to imply that the protagonist is in denial about her unpopular personality.
Early in the story it is indeed abundantly obvious that it to a very large degree is Tomoko's fault she has no friends, as she only too readily acts weird, creepy or plain jerkish - though in spite of the denialist tone of the title she's actually fairly sardonically self-aware about it. This gradually flips the other way as she grows as a person and develops a growing circle of genuine friends (of whom at least three or four seem to have varying degrees of interest in her beyond that, IYKWIM, whence all the "harem" jokes and shipping around the manga) and more distant pals; point being that in all stages the title of the story is A) an ironic and blatant lie B) reflective of themes of the story, mainly the MC's early jerkass tendencies. (The latter never entirely disappear, mind, but start being balanced out by genuine efforts to try and be a decent damn person.)