My Best Friend's (sister) Sibling

Hwang Hyunjin had just moved into a new neighborhood, all on his own. There, he met Lee Yongbok, who quickly became his best friend. Yongbok lived with his sibling, an older brother, Lee Minho.

What happens when Yongbok's younger sister comes to her older brother's?

What happens when Hyunjin starts to take an unusual liking to her?