I stayed over at my girlfriend's place today. We were sleeping together in her room when I suddenly woke up in the middle of the night. Feeling thirsty, I got out of bed and headed to the kitchen. After grabbing a drink of water from the fridge, I heard strange, unsettling sounds coming from the living room. Curious, I went to check it out—and there was her older sister, tightly bound in rope, indulging in a masochistic masturbation session. Unable to resist the temptation, I gave in and began to train her...