From MangaHelpers:

Nomura Moeko's family has been shaken by the death of her older brother. Not very long afterward, Moeko's father makes the decision to take the son of one of his acquaintances into his home. Kiichi is a year younger than Moeko, and Moeko's father intends for the boy to become Moeko's little brother. Moeko doesn't dislike Kiichi, but it is difficult for her to deal with her new 'replacement' brother while she is still coming to terms with her grief. Kiichi, newly come from Tokyo, enjoys his new rural life by the ocean despite the obvious problems of his new family. Of course, life goes on, and as Moeko and Kiichi go to middle school and spend more time together, maybe the slow forging of their relationship will help their new family to be able to heal. [tethysdust]