My Beloved Sister-in-law, Who Raised Me With Her Own Hands, Was Cuckolded By A Lousy Friend... Ai Mukai

Ai and I aren't real siblings. When I was a student, my father remarried, and Ai was the child his new wife brought with her. A year later, after our parents died, Ai became like a parent to me. Living alone together, I began to develop special romantic feelings for Ai, my only family. Just as I swore I would make Ai happy no matter what, my bad friend Shinji started getting close to our family. 'Your sister's pretty damn sexy and beautiful, huh,' he said, and started hanging around my house...