Not sure how 姉さん keeps being rendered as "Ane-san" but, at least for Joon that's not her reading of it. While the kanji can be "Ane", supposedly, the common use (and the way Joon uses it) is "Nee" in this case. Ichirin uses a different kanji for "sister", though given ZUN used it instead of the common one seen here, I am going to assume for that kanji it has an "ane" reading (UFO has no readings for Ichirin's dialogue). Looking around I'm guessing the translator went with "Ane-san" BECAUSE Ichirin/some buddhists (in fanworks at least) call Byakuren 姐さん. Fair enough guess, just likely wrong for Joon who calls her sister Nee-san.
Updated
Nee-san?!
Just now, did you call me Nee-san?!
Is there anything you want to tell your Nee-san?!
BWOOOSH
Ah.
Ah...
over...
Now I've done it...
Oh?
YesyesYESYES, what is it?!
No, there isn't! I mean, no I didn't say that!Yes, you did~ It's quite all right~ There's no need to be shy~Smack
I don't care, dammit!
If you manage to hit me even once, you can consider your training over...