@Enigma absolutely!! I mean they can just choose to keep the original Korean word for sister i.e. 'noona'. Since they use it to address a girl older than them, either romantically or not. Translating it makes it feel weird since the English word 'sister' is not used the same way. Or they can just remove the whole da** thing.
@ihatecliffhanger yeah, that's the least they could do, to add some notes of explanation. But no, they chose to do nothing. Maybe they like watching us wreck our brains on this.