My innocent girlfriend's younger sister missed the last train and came to stay the night. (Kuraki Hana, SONE-398)

A week ago, I had dinner with her and her sister. "I skipped the last bus, can I come over now?" I received this call from her sister, whom I'd only met once. "I didn't tell my sister, so she won't find out, right?" Everything she said and did made me think she was seducing me. I...I...I...can do it (swallows). We were only 10 seconds away from a forbidden relationship...