Watching her boyfriend fuck her sister but not being able to do anything

Mizuki is not a real sibling. When I was a student, my parents remarried and we got together. Several years have passed since then, and after their parents died in an accident, they lived together and supported each other. As we continued to live alone, I began to love Mizuki. The dream I imagined was to be with Mizuki when it was stolen by my best friend Kyohei. Kyohei, who liked Mizuki, was jealous of our relationship, and suddenly approached Mizuki when I wasn't around …