Seems like Mu just doesn't acknowledge Sora yet. I suppose, to be fair, Touhru has been feeding her unknowingly for a while already, so she probably trusts him a bit more.
more so, English has gender neutral singular terms in 'they' and 'their'. It's relatively recently that an English teacher decided the practice of assuming male gender but for most of the language's history, that practice didn't exist. It's another one of those "proper English is a bullshit idea made by someone trying to make it like Latin because Latin is a 'superior language'" kinda things.
also, doubt anything is going to happen to her. She's still on the later covers.