Based on chapter 6 (Love-Hate-Really Love) of the manga Ane-Koi by Yuzuki N'.

A love story between Nee-chan named Sana and his younger brother named Yoshiharu, starting from Yoshiharu who daily sees his Nee-chan underwear every sleeping on the couch and starts to lust for the beauty of his own Nee-chan body.

(Source: Nekopoi, translated)