This might actually be a case of Transcription Error. The Suethor might have written that part of the story by hand (in the back of mom's minivan while on a long trip, say?) and then given it to Big Sister to type up while she was, oh, let's say taking a shower to get ready for the first day of middleschool. And her handwriting was so abysmal that Big Sis honestly thought that "Sirius" was "Sinus".

This is why I've stopped letting anyone try to read my writing while it's still in handwritten form. My "t"s look rather more like "d"s... And I can honestly see how "ri" can look like "n"....

Still, proofreading is inexcusable.

...is that someone wasn't paying enough attention when they ran the spellchecker over. (No, 'Change All' is not a good option).