Inmai Seikatsu ~Ore wa Imouto de Shika Tatanai~

The protagonist and his little sister Chika felt special about each other, but their first attempt at sex was awkward and the mood soured. He holed up in his bedroom to look at porn. Timidly she poked her head in. It was just a lover's quarrel. Now that they weren't angry at each other she offered to an apology: "While you're looking at that, would you like me to get you off?"