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.
Well... there are some titles that have nothing to do with the work... and there are others that describe their work waaaay too accurately... That's the kind of thing I learned from the only 2 Gigguk videos I watched.
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.
From what I understood from the fanarts and comments to them, the protagonist (I think her name is Tomoko? Correct me if I'm wrong) isn't at all popular with the boys at her school, but is very popular with the girls. I remember one commenter to one of those fanarts half-jokingly saying that the manga should be renamed "It's not my fault I'm a chick magnet!"
