It’s because it’s translated, theirs no English equivalent to what the word really means which is just another female around your age but the closest English word to it is sister. When they use it they mean a girl close to age with herself not a sibling or actual sister. I read a lot of translations so I’m issued to it so just reading it more you’ll understand what they mean the word as
Tysm for clarifying
I mean ya but they literally said that the mc was the FL’s step sister .
If they were talking about an girl that close in their age range they wouldv’e js clarified that instead of saying that the MC and FL was step sisters .
was the sister's bit rlly necessary