Watashi-tachi no Koto wa Nanto Yobeba

The manga's story begins in high school with Mineko and Sunao, who start a romantic relationship with each other. However, their single parents remarry each other, turning them into brother and sister. Mineko and Sunao mutually agree to suspend their relationship in favor of their new familial connection. But a tragic accident some years later leads to both of their parents' deaths, and now they live under one roof as responsible, independent adults, with the memory of their romance still hidden in their hearts.

(Source: Anime News Network)