Is this 'onee-san' address sarcastic or something?

Question

S01E03 of anime adaptation of the LN/manga The Devil Is a Part-Timer!

See here 1:18 - 1:32 There are these 2 characters Chiho and Emi who meet for the 1st time when Emi interrupts Chiho's profession (confession?) of romantic feelings to Maou, who is Chiho's seniour colleague and the title character. Chiho assumes Emi is Maou's ex-romantic partner. In the part translated as 'It's none of your business now!' Chiho seems to address Emi as onee-san.

1. Is that in like a sarcastic way or something? I thought Chiho would say something like 'omae' ? Or maybe it's a slight in pointing out how much older Emi is?

2. Actually, I notice the English dub has a non-corresponding part (see here 1:14 - 1:19) where Chiho says 'If anybody here needs to stop yakking, it's you, sister!' So is the meaning of 'onee-san' by original Chiho similar to the meaning of 'sister' by dub Chiho?

Note 1: In the English dub, 'sister' is not directly translated from onee-san because in the original Chiho is asking Emi 'Are you trying to say you're more intimate with Maou-san?' The 'sister' is actually 12 seconds (1:14 dub which is 1:40 original vs 1:28 original) after the use of 'onee-san' in the original.

Note 2: I think the dub is changing the dialogue vastly like trying to make it more of a romantic fight when in the original they're trying to understand what the other is saying.

Answer 1

お姉さん ("onee-san") is not only used to mean "sister". In common speech it is quite often used (particularly by kids/young adults) when talking to any young woman who is older than them (particularly if they don't know her name). It is often similar to saying "miss", etc, in English. (the same is true with お兄さん ("onii-san") and guys)

In this case, it is really just a more respectful way to say "you" to a girl who is older than her (in this sort of situation, using あなた, etc, would probably come off as pretty rude, and as I remember, Chiho didn't actually know Emi's name at this point, so she wouldn't be able to use her name).

Likewise, おばさん and おじさん are often used to address somewhat older men/women in general (basically similar to "ma'am" and "mister"), and おばあさん and おじいさん are sometimes used to address old ladies and old men (one should be very careful about the pronunciation of おばさん and おじさん, though, because if you hold the vowel a little too long, you can end up calling someone "old woman" when you meant to just say "ma'am", etc (there are many tropes about young kids making this mistake and (vain) women getting annoyed at being called "granny" before their time, etc))

Answer 2

I have a different answer: Chi says onee-san twice before motokano. Hence, it's not sarcastic. Lol.