Little sister is ripe, and the plan is to force while the gf is away. For 2 days while she’s away, relentlessly seeded and raped. Yuuhi Shidara

The protagonist, Hajime, is introduced to his girlfriend’s sister and gets csaptivated by her beauty. From that day on, he begins to channel his inner psychopath. When his girlfriend leaves for a few days, he takes the opportunity to spend some alone time with his sister.