Love Is Indivisible by Twins

恋は双子で割り切れない

Jun Shirosaki moved in next door to the Jinguuji twins at six, and the two households have functioned as one family ever since. Rumi, the older sister, asked him out in middle school and then broke it off herself; Naori, the younger, has never stopped competing for him. By high school the three are stuck: Rumi hides a sentimental streak behind a tomboyish front, Naori's cute-otaku act covers just as much, and Jun cannot bring himself to choose between them. This light novel is the original source for a manga adaptation of the same story.