Or Something Like It

Suddenly, her life was turned upside-down.... Kinaho lived alone until one day, her boyfriend Akito brought with him two young relatives: a pair of brothers who had just lost their parents. All at once, Kinaho finds herself living under one roof with not just Akito, but serious-to-a-fault eighth grader Toma and innocent five-year-old Haruhi. As the days pass in confusion, this group starts to forge a new relationship: a family... or something like it.

(Source: MangaPlaza)