Haha, I totally understand. I don’t trust those guides at all.

I’m a language lover myself (I like learning, but after trying for many years with multiple languages, I’m not super into practice ;) so I learn about how languages work instead!) and if there’s anything I’ve learned about language it is this:

It does not matter how you sound or what you actually say as long as the message you intended to get across actually gets across to whomever you mean to hear it. If people mispronounce, it is usually either regional (and thus correct for them) or something they read and have never heard anyone say. If they use the wrong word but it’s kinda right, they are probably language learners.

This was galvanized for me when I took an art history class as a general education credit in college. I learned that clerestory is pronounced clear-story. I’d only ever read the word before that, and thought it was more in line with modern patterns to be CLE-rest-ory, which is embarrassingly wrong. I’d been reading it that way for years.

Your sister sounds like a language prescriptivist, and they are always wrong, because language simply doesn’t work like that.