I reckon this song is about someone who is probably a pretty close friend of the person singing. Most likely someone they grew up with and so is like a sister to them (but not an actual sister). The 'sister' is most probably a few years younger, and the singer grew up and fell in love with her, but she grew up and he never told her, though it looks like she probably fell in love with someone else, then she left.
I reckon this song is about someone who is probably a pretty close friend of the person singing. Most likely someone they grew up with and so is like a sister to them (but not an actual sister). The 'sister' is most probably a few years younger, and the singer grew up and fell in love with her, but she grew up and he never told her, though it looks like she probably fell in love with someone else, then she left.