MIAA-384 - Stranded by the heavy rain, my girlfriend's younger sister and I end up crossing the line in the dark

I was planning to marry my girlfriend, but her younger sister, Rin, always had a way of getting close to me. One stormy night, she showed up at my place soaking wet. When a sudden power outage plunged us into darkness, she clung to me in fear. The moment I felt her body against mine, I lost all control. Overcome with guilt and desire, we couldn't stop ourselves—from the bathroom to the bedroom, we spent the entire night indulging in each other.