i haven't read this yet but im pretty sure it's honorifics. In China people call people a bit older than them older sister or older brother.
oh no i know that but she knew him when they were younger cause he's like "sister forget me" Also i have a bit of an overdrive mind so I assume FL and the little bro were in a fire so they both lost their parents and FL and little bro end up in an orphanage and FL got psychological scar so she forgets everything that's what my mind was thinking that's why i was confused if they real bro and sis
They are neighbors
weird so are they really bro and sis? or what this is kind of weird, i need some spoilers ASAP