I really hope is about the sister understanding the value of falling in love by both seeing her sister and the boy fall in love, as a way of seeing a version of herself and possibly that other guy from the extra and what could have been, and not that she will actually develop romantic feelings for a MINOR as her TEACHER. I'm really not into those type of plots, it's weird and not ok, i don't care how much they try to normalize or romanticize it.