My Beautiful Girlfriend Cheated on Me, so I found Comfort in Having a Relationship with Her Little Sister

One day, Kuroyami Koichi finds out that his girlfriend, Shirahara Mana, has been cheating on him with another man. On the other hand, Shirahara Yume, Mana’s little sister, secretly habors feelings for Koichi. Knowing this, Yume makes the move to get closer to Koichi, and plans to take Koichi away from her older sister