Sokubaku Suru Ane

Due to their father’s job transfer, the siblings ended up living apart from their parents. One day, the older sister caught her younger brother in the act of masturbating. To make matters worse, in his hand was… her panties. That’s right—he’d been using the panties of his beloved older sister, Satsuki, to get himself off. He braced himself for her to yell at him, but instead, she gave him a surprisingly gentle smile. However, the words that followed were even more unexpected… "Starting today, no masturbating or ejaculating without my permission, okay?" [Translated from dlsite]