Ane to Ore to Senpai to

High schooler Akio arrives home early one day, and, much to his shock upon opening the door, finds another girl pinned under his older sister, Touko. She reveals that she's only into girls, echoing his own feelings of attraction to the same sex, but he hesitates to show his support, having struggled to escape from living in her shadow all his life. That very same Akio ends up falling in love with the most popular guy in school, Yoshito. The catch is, Yoshito's only nice to him is because he has his eyes on his sister... Source: MU