Ohhhhh.... I misunderstood somethin’, he said that she was LIKE a younger sister.#-.-) 。WintersAreCold。
She is a neighbour who calls him Oppa. Though it means "older brother" in korean, it can really mean any older guy (related or not) of the same generation.
I think I threw up a bit inside when I saw the title....Σ(  ̄□ ̄||)