Ane Kyun: Joshi ga Ie ni Kita!

A sister named Honoka who likes her younger brother Hirokazu, that Honoka sees Hirokazu doing schoolwork with a woman named Akazaki, and she feels jealous. Then he ventured to declare his love and he really wanted a Sex Relationship with Hirokazu. But Akazaki heard a voice from the next room and he suspected Honoka was watching a porn scene. And the next scene is transformed into a sex between Honoka and Akazaki.

(Source: Nekopoi and Has Been Translated and Edited)