FNS-214 - Wait, who are you? I deep-kissed my girlfriend, but it was actually her little sister

I snuck into my girlfriend's place while her family was away, ready for a wild night. I went in for a deep, passionate kiss, but when I finally got a good look, it wasn't my girlfriend—it was her little sister, Sana! I thought I was doomed for stealing her first kiss, but the next morning, she showed up with a flushed face and whispered, 'Kiss me again,' luring me right back into the trap.