I took French for four years in high school (in Ontario) and have never heard of the “si” usage for “oui.”
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Hmmm. Wonder if it’s not used in French Canadian or something. It was part of our French lessons, but we learned Continental French (or whatever you want to call it.) It’s not really “si” for “oui,” though. It’s only used in specific cases.
What’s worse is in some Indian dialects, “yes” in response to a question doesn’t mean that the answer is yes, it means that the responder heard the question.
A similar thing happens in American English too, although usually with a bit less ambiguity, mostly due to intonation and the like. Then, there’s the “yes, no” phenomenon that some people don’t like.
The use of “si” as “yes” to a negative question is rare in Québécois French. In those cases, they use “oui” (as in English) unless it’s a particularly formal situation or there’s some particular need to avoid ambiguity.
That explains it. About all we learned about Quebecois French (and I think maybe Swiss French) was their more straightforward numbering nomenclature for words like 70, 80, and 90. 
Eh? In Canadian French, we use the soixante-dix, quatre-vingt, and quatre-vingt-dix.
I was also taught the difference between “oui” and “si” in my little anglo school on the prairies.
♫ Oui, nous n’avons pas de bananes, nous n’avons pas de bananes aujourd’hui. ♫
Whoops. Then the Swiss French. Whoever uses the septente, huitante, nonante, etc.
English still has this distinction. You can’t just answer “yes” to refute a negative statement. You say “yes I do”, “yes he is”, “in fact yes”, or something similar. You’re just using a whole sentence instead of a single word.
In Switzerland and Belgium.
I only heard those terms when speaking with a francophone Swiss-Canadian.
My (mildly autistic) 8-year-old also answers negative questions “backwards” and cannot be persuaded that it is incorrect. Actually, my 12-year-old occasionally does it, too.
They have almost managed to condition me and my husband to only ask questions in positive form.
The word cherā is literally ‘why?’. It’s che ‘what’ + -rā (object suffix). Basically, ‘What for?’ If somebody says a negative clause and you disagree, you say something like “Whatchoo talkin bout, Willis?” It’s natural pragmatics (in the linguistic sense).