It's exactly what the title says. 

Chaemin has alwas been jealous of her elder sister, Jihae. Everytime Chaemin finds someone she can get close to, Jihae can always slowly snatch that person away from her.  When their parents decided to move to the city, they enrolled to a new school. It was then Chaemin met a boy at her age named Zelo. Chaemin grew to like the boy and thought that maybe, just maybe, this time it'll be different. But as the days go by, as the usual, Zelo starts to pay more attention to Jihae.