See here 1:18 - 1:32 Chiho and Emi meet for the 1st time when Emi interrupts Chiho's profession (confession?) of romantic feelings to Maou. 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.
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?
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.